Map from Google - Philippines

In the Philippines, an archipelago of thousands of islands, the climate is generally tropical maritime, with a relatively cool season from December to February, when the north-east trade winds prevail, and a hot, humid and rainy season from May to November, in which the south-west monsoon prevails. Between March and May, before the arrival of the summer monsoon, the temperature increases and reaches the highest levels of the year, especially in the center-north and the interior of the larger islands: hence, in general the coolest month (or the least hot) is January and the warmest is May.
In the north of Luzon, from December to March cool air masses may sometimes arrive, so much so that at night the temperature can drop to around 54/59 °F. In the southern islands, close to the Equator, the temperatures are more stable, and remain high all year round.
Even the relative humidity in the Philippines is almost always high, and makes the heat muggy, at least in the plains.
Some areas (usually the eastern coast, facing the Philippine Sea), have an equatorial climate, ie rainy throughout the year. In fact, during winter the northeast trade winds discharge a large amount of moisture in the form of rain on exposed coasts and slopes. This in general rarely happens in the inner islands, which are more sheltered, and even less along the western slopes, where winter is a dry season.
The tropical rains typically occur in the form of downpour or thunderstorm in the afternoon, except during tropical storms and typhoons, which can last for a few days and are accompanied by strong winds.

Equatorial climate

Philippines - areas with equatorial climate

In summer it rains all over the Philippines; the equatorial climate zones are those in which it rains even in winter, that is, those exposed to the northeast winds. They are the eastern part of the island of Luzon, some of the Eastern Visayas, like the island of Samar and the eastern part of Leyte, the north-east of Mindanao and the smaller southern islands. In these areas, more than 4 inches of rain per month fall even in winter, which is often the rainiest season. In fact these areas, being exposed to the easterly winds, are generally at least partially sheltered from the summer monsoon, which as mentioned blows from the south-west. In these areas the annual precipitation exceeds 100 in, but throughout most of the easternmost coast it exceeds 120 in, and in some cases even 160 in
Here is the average precipitation of Legazpi, located in the south-east of the island of Luzon.
Average precipitation - Legazpi
LegazpiJanFebMarAprMayJunJulAugSepOctNovDecYear
LegazpiJFMAMJJASONDY
Prec. (in)11.67.77.75.97.19.49.47.18.510.419.118.1122
Days191515151416171617202222208

Here are the average temperatures of Legazpi.
Average temperatures - Legazpi
LegazpiJanFebMarAprMayJunJulAugSepOctNovDec
LegazpiJFMAMJJASOND
Min (°F)727273757777757575757575
Max (°F)848486889090909090888684

Being rainy throughout the year, these areas of the Philippines honestly do not have a beautiful climate, and not surprisingly here you won't find large cities.
The least rainy period varies depending on area, and advances in the year proceeding from north to south. In Legazpi (Luzon Island), less than 8 inches per month fall from March to May, while in Borongan (Samar island) this happens from July to September.
The southernmost islands (Basilan, Sulu, Tawi-Tawi), which lie north of Borneo, must be put into the equatorial climate zone, with rainfall even in winter, but since they do not have an evident peak in this season, they can be visited from February to April, and after all even from June to September, when however it rains on average every two days.
Here is the average precipitation of Bongao, in the province of Tawi-Tawi.
Average precipitation - Bongao
BongaoJanFebMarAprMayJunJulAugSepOctNovDecYear
BongaoJFMAMJJASONDY
Prec. (in)8.36.95.35.98.37.56.75.75.98.57.98.385
Days141111101316151515171616169

Tropical climate

Philippines - areas with tropical climate

In the rest of the Philippines, the climate can be defined as tropical, since it is possible to identify a dry season, which coincides with winter, and is most evident in the western part (blue area), and less evident, but still present, in the central part (green area), while the summer monsoon is generally more intense in the western part.
In this area, the biggest cities of the Philippines are found, starting from the capital Manila.
In the northernmost islands (Batanes and Babuyan), which are in the intermediate area circled in green, it's pretty cool from December to February, with average highs around 75/77 °F, because they are located far to the north, almost to the Tropic, and are affected by cool air masses from the Asian continent.
Here are the average temperatures of Santo Domingo de Basco, in the Batanes province, in the far north of the Philippines.
Average temperatures - Basco
BascoJanFebMarAprMayJunJulAugSepOctNovDec
BascoJFMAMJJASOND
Min (°F)666870737777777775737270
Max (°F)757781828888888886828177

The wettest period is from July to December, but even January is often very rainy, in fact in this month sometimes the rains still amount to 7 in. It goes a bit better in February, but the driest months here are March and April, so they will be the ones to be preferred for a visit.
Here is the average precipitation.
Average precipitation - Basco
BascoJanFebMarAprMayJunJulAugSepOctNovDecYear
BascoJFMAMJJASONDY
Prec. (in)7.14.93.52.86.710.21116.5151311.610112.2
Days18141081014152018181919183

In winter, the sea remains warm enough for swimming even in this northernmost part of the Philippines, although the air can be a bit cool.
Sea temperature - Basco
BascoJanFebMarAprMayJunJulAugSepOctNovDec
BascoJFMAMJJASOND
Sea (°F)777779818284848484828179

In the interior of Luzon, the pre-monsoon heat from March to May is fairly noticeable: there are very hot city like Angeles, San Fernando, Tarlac, Cabanatuan and Tuguegarao, which in this period easily reach 95/100 °F, but with peaks of 105 °F.
Average temperatures - San Fernando
San FernandoJanFebMarAprMayJunJulAugSepOctNovDec
San FernandoJFMAMJJASOND
Min (°F)686872737373737373737270
Max (°F)889093959593909090909088

The metropolitan area of Manila and Quezon City is located in a better position, because it is situated on the coast and therefore gets some breezes, however, being a big city, the so-called urban heat island effect is evident. In Manila and Quezon City, it's hot all year round, since the maximum temperature is around 86 °F even in winter. Here are the average temperatures.
Average temperatures - Manila
ManilaJanFebMarAprMayJunJulAugSepOctNovDec
ManilaJFMAMJJASOND
Min (°F)757577798179797979797775
Max (°F)868890939190888888888886

The dry season, from January to April, is very pronounced: in general, it almost never rains, especially in February and March, which are the driest months. The hottest months are April and May, in which on average the temperature is around 92/93 °F during the day, with peaks of 97/100 °F. In fact, high temperatures like these are possible even in the rainy season, albeit more rarely. During the rainy season, from June to October, the average temperature drops, so that highs stay around 88/90 °F, although lows are high, around 78 °F and the humidity makes the heat muggy. The rainiest months is August, with about 19 in of rain. Here is the average precipitation.
Average precipitation - Manila
ManilaJanFebMarAprMayJunJulAugSepOctNovDecYear
ManilaJFMAMJJASONDY
Prec. (in)0.60.30.80.85.511.214.418.713.27.94.32.279.7
Days4233101622222018149143

The amount of sunshine is never scarce even in the wettest months, because usually in the morning the weather is good, while in the afternoon or in the evening, showers and thunderstorms often occur.
The best months to visit Manila are January and February, being the least hot among those of the dry period.
At Manila, the sea is warm all year round, with little variation over the months, and this applies also to the rest of the country.
Sea temperature - Manila
ManilaJanFebMarAprMayJunJulAugSepOctNovDec
ManilaJFMAMJJASOND
Sea (°F)818182848686868484848482

Manila

Cebu City is located in the central group of islands (Visayas), which are more sheltered, so they are moderately rainy but without excesses. Here, about 53 in of rain per year fall, of which less than 4 in from February to May, and 8 inches in October, which is the rainiest month. For many months of the year, it rains on average every second day, except during the period from February to May. We are in the intermediate zone (green), where the dry winter period is present but less pronounced, January is still a pretty rainy month, and therefore the best period is from February to April. Here is the average precipitation.
Average precipitation - Cebu
CebuJanFebMarAprMayJunJulAugSepOctNovDecYear
CebuJFMAMJJASONDY
Prec. (in)4.32.80.81.22.86.77.14.36.37.94.34.352.8
Days14996813151314151214142

Davao City is located in the south of Mindanao, overlooking the gulf of the same name, and at a short distance from mountains such as Apo and Leonard Kniaseff. Here we are at the boundary between the tropical and the equatorial climate zone; the annual rainfall amount is 80 in, and there's not a well-defined dry season, although between December and March, the level of precipitation drops to around 3.5/4.5 inches per month.
The least rainy area of the Philippines is the protected Sarangani Bay, in the far south of Mindanao. In General Santos, 40 in of rain per year fall, including about 4 inches per month, therefore not very much, in the summer months.

Mountains, typhoons, El Niño

In the Philippines there are many mountains and volcanoes, the highest of which is the aforementioned Mount Apo, in Mindanao, almost 10,000 feet high. Along the mountain slopes, usually the rains are more abundant than in the plains, and as mentioned, in the exposed ones, showers and thunderstorms are possible throughout the year.
In Baguio, at 5,000 feet above sea level on the island of Luzon, 162 in of rain per year fall; the rains are plentiful especially in July and August, with almost 40 inches of rain per month! Here is the average precipitation.
Average precipitation - Baguio
BaguioJanFebMarAprMayJunJulAugSepOctNovDecYear
BaguioJFMAMJJASONDY
Prec. (in)0.80.824.713.816.536.239.225.614.26.32.2162.2
Days44610202327272519106181

However, there is little rain from December to March, when Baguio, with its mild temperature due to the altitude, is a haven to escape the heat of the cities of the plains, especially in the hottest period of the year, ie in March and April, before the rainy season (which here begins already in May).
Average temperatures - Baguio
BaguioJanFebMarAprMayJunJulAugSepOctNovDec
BaguioJFMAMJJASOND
Min (°F)545557596161595959595955
Max (°F)727275777573707070727373

The Philippines are struck in full force by tropical cyclones, which in south-east Asia are called typhoons. The only area which generally remains outside of their trajectory is the southernmost, therefore the island of Mindanao, especially in the south-central part, and the small southern islands. The typhoon season runs from mid-May to mid-November, although they are more frequent between August and October. However, they can sometimes arrive late, as happened with typhoon Bopha, which affected the Philippines on early December 2012, or with Hagupit, which struck the islands around December 10, 2014.

The Philippines are also affected by the phenomenon known as El Niño, which in some years upsets the climate of large areas of the world, and in the Philippines brings heat, drought and fewer typhoons than usual, while the opposite phenomenon, La Niña, brings heavier rainfall than normal and slightly lower temperatures.

When to go

The best time to visit the Philippines as a whole runs from February to April, because in the regions with an equatorial climate, which in any case are rainy all year round, January is a particularly rainy month. Since March and April are very hot months, February is the best month of all. In areas with a tropical climate, where the winter drought is evident, such as Manila, January is also a good month, and is often the best, because it is dry and also the least hot.
Wanting to travel in July or August, you will prefer Mindanao and the southern islands (Basilan, Sulu, Tawitawi), which are less rainy and therefore are acceptable even in this period, and most of all are generally sheltered from typhoons.
As mentioned, the sea is warm enough all year round for swimming.

What to pack

In winter: in the northernmost islands (Batanes and Babuyan): spring/autumn clothes, light for the day, sweatshirt or sweater, jacket, raincoat or umbrella. In the north of Luzon, light clothing, a sweatshirt or sweater for the evening, raincoat or umbrella. In Manila and the rest of the islands, light clothing, made of natural fibers, a scarf for the breeze, a light sweatshirt for air-conditioned places, light raincoat or umbrella for the areas where it rains even in winter. In Baguio, at 5,000 feet, spring/autumn clothes, light for the day, jacket and sweater for the evening. For the mountains, warmer clothes depending on altitude.

In summer: throughout plains and the coasts, tropics-friendly clothing, a scarf for the breeze, a light sweatshirt for air-conditioned places, light raincoat or umbrella. In Baguio, at 5,000 feet, spring/autumn clothes, jacket and sweater for the evening, raincoat. For the mountains, warmer clothes depending on altitude.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.