Map from Google - Guyana

In Guyana, which overlooks the Atlantic Ocean in the north and touches the Equator in the extreme south, the climate is hot and humid all year round, with two rainy seasons (from December to early February and from late April to mid-August), and two relatively dry seasons (typically February-March and September-October). The duration and intensity of rainfall vary depending on area, but everywhere the period May-August is rainy. In the areas which don't have a real dry season, we can speak of equatorial climate, otherwise of tropical climate. The rains occur mainly in the form of heavy showers and thunderstorms, usually in the afternoon or evening.

In the north-west (see Mabaruma), there is not a real dry season, but only a relative decrease in precipitation: here even September and October are quite rainy, around 200 millimetres (8 inches) per month, while the decrease in the rainfall from February to mid-April it's more noticeable, when around 100 mm (4 in) of rain fall per month.
Here is the average precipitation.
Average precipitation - Mabaruma
MabarumaJanFebMarAprMayJunJulAugSepOctNovDecYear
MabarumaJFMAMJJASONDY
Prec. (mm)195851051602804103602552102152953002875
Prec.(in)7.73.34.16.31116.114.2108.38.511.611.8113.2
Days181011101923252413181720208

Along the central and eastern coast (see the capital Georgetown), comparing the two drier seasons, the period from September to October is only slightly less rainy than the period from February to March, so the difference is not significant. In Georgetown, 2,400 mm (94.5 in) per year fall. Therefore, here too there is no real dry season. The wettest month is June, with 345 mm (13.5 in) of rain.
Here is the average precipitation in Georgetown.
Average precipitation - Georgetown
GeorgetownJanFebMarAprMayJunJulAugSepOctNovDecYear
GeorgetownJFMAMJJASONDY
Prec. (mm)25012011518029534528018590951503152420
Prec.(in)9.84.74.57.111.613.6117.33.53.75.912.495.3
Days1713121220232116791020180

In the easternmost part of Guyana, near the border with Suriname, the second dry season is longer and a bit more pronounced, and includes even the month of November.
Here is the average precipitation in New Amsterdam.
Average precipitation - New Amsterdam
New AmsterdamJanFebMarAprMayJunJulAugSepOctNovDecYear
New AmsterdamJFMAMJJASONDY
Prec. (mm)22514013518028529516516560701002502175
Prec.(in)8.95.55.37.111.211.66.56.52.42.83.99.885.6
Days20121214191818159101219178

In the south-west, towards the border with the Brazilian state of Roraima (see Lethem), we find the area of the savannah, where the climate is tropical, with a dry season which runs from October to March, in which the heat is intense, with highs around 33/35 °C (91/95 °F).
Here are the average temperatures of Lethem.
Average temperatures - Lethem
LethemJanFebMarAprMayJunJulAugSepOctNovDec
LethemJFMAMJJASOND
Min (°C)232324242423232323252424
Max (°C)323333323130303133343333
Min (°F)737375757573737373777575
Max (°F)909191908886868891939191

Although the total quantity of rainfall is high, around 1,700 mm (67 in), here there is a single and long dry season, with poor rains, around 30 mm (1.2 in) per month from November to March, and 50 mm (2 in) in October. In April and September the rains are moderate, while they are heavy from May to August.
Here is the average precipitation.
Average precipitation - Lethem
LethemJanFebMarAprMayJunJulAugSepOctNovDecYear
LethemJFMAMJJASONDY
Prec. (mm)303030110300375340275805035251680
Prec.(in)1.21.21.24.311.814.813.410.83.121.4166.1
Days656102123222111965145

In addition to the strip along the border where Lethem is located, called North Rupununi Savannah, there is a more extensive area of savannah, called South Rupununi Savannah, located south of the Kanuku Mountains (a low mountain range covered by forests).

North of the Rupununi Savannahs, there's a mountain range, the Sierra Pacaraima, culminating in Mount Roraima, whose top is in Venezuela, but in the Guyanese part it exceeds 2,200 metres (7,200 ft); other peaks are Mount Ayanganna, 2,041 metres (6,696 feet) high, and Mount Caburaì, 1,465 metres (4,806 feet) high. Obviously, the temperature decreases with increasing altitude, and although we are near the equator, above 1,500 metres (5,000 feet) the air becomes cool. On the northern slopes of this mountain range, precipitation reaches 3,500 mm (138 in) per year. In these mountains, the Potaro River has its source, and then it feeds the huge and spectacular Kaieteur Falls.

Kaieteur Falls

We have said that the climate of Guyana, at least in the plains, is hot all year round. In the capital Georgetown and in the rest of the country, the temperature is fairly constant throughout the year, with a minimum around 24/25 °C (75/77 °F) and a maximum around 29/31 °C (84/88 °F). The daytime temperatures are slightly higher in the September and October, and slightly lower in the period from December to March. Here, as in general along the coast, during the day the heat is tempered by sea breezes.
Average temperatures - Georgetown
GeorgetownJanFebMarAprMayJunJulAugSepOctNovDec
GeorgetownJFMAMJJASOND
Min (°C)242424252524242425252424
Max (°C)292929303029303031313029
Min (°F)757575777775757577777575
Max (°F)848484868684868688888684

The sea in Guyana is warm all year round: the water temperature ranges from 26 °C (79 °F) to 28 °C (82 °F).
Sea temperature - Georgetown
GeorgetownJanFebMarAprMayJunJulAugSepOctNovDec
GeorgetownJFMAMJJASOND
Sea (°C)272627272727282828282827
Sea (°F)817981818181828282828281

The amount of sunshine is not high in the rainy months, while it is good in the dry periods, when the sky is usually clear, even though a bit of cloudiness may develop at any time.
Guyana is outside of the path of hurricanes, which pass to the north, on the Caribbean Sea.

When to go

The best time to visit Guyana depends mainly on the rain pattern, which as we have seen is not the same throughout the country. In the north-west (see Mabaruma), the best period is February-March. In Georgetown and in the central area, the best time is September-October, but also February-March. In the east, the best period is September-October, but it's extended until November (and thus becomes September to November). In the south-western area of the savannah (see Lethem), you can go from October to March.
Having to choose a single period for the whole country, you can choose February and March.

What to pack

Throughout the year: lightweight clothing of natural fibres, possibly with long sleeves for mosquitoes; a light sweatshirt; sun hat; umbrella, or lightweight and breathable raincoat. For mountain climbing, hiking shoes, a sweatshirt.
From December to March, a sweatshirt for the evening.