Map from Google - Israel

The climate in Israel is characterized by hot and sunny summers, and by a long period of sunshine, with clear skies from May to September. The rains (where they occur) are concentrated from November to March.
Along the west coast (see Tel Aviv, Haifa, Netanya), the climate is Mediterranean, with very mild and rainy winters, and hot and sunny summers, during which the air is humid but tempered the breeze. In Tel Aviv, the average temperature in January is 14 °C (57 °F), while in July and August it's about 26 °C (79 °F). Heat waves can occur especially in the period from April to June, when the wind from the Egyptian desert can raise the temperature to around 40 °C (104 °F) even on the coast.
Here are the average temperatures in Tel Aviv.
Average temperatures - Tel Aviv
Tel AvivJanFebMarAprMayJunJulAugSepOctNovDec
Tel AvivJFMAMJJASOND
Min (°C)101012141720232322191512
Max (°C)181820232527293029272420
Min (°F)505054576368737372665954
Max (°F)646468737781848684817568

Rainfall, concentrated in the winter months, amounts to around 580 millimetres (23 inches) per annum, and from December to February it is quite abundant. Here is the average precipitation.
Average precipitation - Tel Aviv
Tel AvivJanFebMarAprMayJunJulAugSepOctNovDecYear
Tel AvivJFMAMJJASONDY
Prec. (mm)1451106015400013580125585
Prec.(in)5.74.32.40.60.200001.43.14.923
Days13118210000471056

The sun in Tel Aviv often shines even in winter, except for the periods of bad weather, while in the long dry season the sky is normally clear.
Sunshine - Tel Aviv
Tel AvivJanFebMarAprMayJunJulAugSepOctNovDec
Tel AvivJFMAMJJASOND
Sun (hours)67891112121210986

On the Mediterranean coast, the sea is warm enough for swimming from June to November.
Sea temperature - Tel Aviv
Tel AvivJanFebMarAprMayJunJulAugSepOctNovDec
Tel AvivJFMAMJJASOND
Sea (°C)181717182124262827262320
Sea (°F)646363647075798281797368

In the interior (see Jerusalem), the climate becomes more arid, but also more continental, due both to the greater distance from the sea and the elevation: Jerusalem is located at 700 metres (2,300 feet) above sea level, Bethlehem, in the West Bank, at almost 800 metres (2,600 feet), while Nazareth is located at a lower altitude, around 300/500 metres (1,000/1,600 feet). In these inland areas, winter is still fairly mild, but intrusion of cold air masses from Russia can bring even snow in January and February (and sometimes also in early March), usually above 600 metres (2,000 feet). During summer, the air in this area is dry and the heat is usually tolerable, but the sun hits hard, moreover, hot air masses from the Egyptian desert can sometimes bring some days with scorching heat even here. In Jerusalem, the average temperature in January is 9 °C (48 °F), while in August it's 24.5 °C (76 °F). Here are the average temperatures in Jerusalem.
Average temperatures - Jerusalem
JerusalemJanFebMarAprMayJunJulAugSepOctNovDec
JerusalemJFMAMJJASOND
Min (°C)66813161819201917128
Max (°C)121315222528292928251914
Min (°F)434346556164666866635446
Max (°F)545559727782848482776657

Annual precipitation is around 550 mm (21.5 in), concentrated in the cold half of the year, while from May to mid-October it almost never rains. Here is the average precipitation.
Average precipitation - Jerusalem
JerusalemJanFebMarAprMayJunJulAugSepOctNovDecYear
JerusalemJFMAMJJASONDY
Prec. (mm)1351209525300001560105555
Prec.(in)5.34.73.710.100000.62.44.121.9
Days12129410000371159

In Jerusalem too, the amount of sunshine is good in winter, while it's excellent from May to September.
Sunshine - Jerusalem
JerusalemJanFebMarAprMayJunJulAugSepOctNovDec
JerusalemJFMAMJJASOND
Sun (hours)67791113121210986

Jerusalem

Further to the east, in the Jordan valley, where the altitude goes even below sea level (from Lake Tiberias to the Dead Sea), winter is mild, but summer becomes definitely hot. Here are the average temperatures in Tiberias, on the shores of the lake of the same name (or Sea of Galilee).
Average temperatures - Tiberias
TiberiasJanFebMarAprMayJunJulAugSepOctNovDec
TiberiasJFMAMJJASOND
Min (°C)101012151922252523211612
Max (°C)181923283336383836322620
Min (°F)505054596672777773706154
Max (°F)64667382919710010097907968

In Tiberias, an average of 435 millimetres (17 inches) of rain fall each year, concentrated as usual in the winter months.
Average precipitation - Tiberias
TiberiasJanFebMarAprMayJunJulAugSepOctNovDecYear
TiberiasJFMAMJJASONDY
Prec. (mm)10590552050001155095437
Prec.(in)4.13.52.20.80.200000.623.717.2
Days12108310000361053

Continuing south and heading towards the Dead Sea, precipitation becomes scarcer, and the landscape, apart from the areas where the river's waters are used to irrigate fields, becomes semi-desert.
Further to the south, in the areas bordering the Sinai Peninsula and the Red Sea, we find the Negev Desert, which has mild winters and hot summers. Unlike the Mediterranean coast, summer is not the best time for a beach holiday at the Red Sea: in Eilat, the average maximum temperature in July and August is 40 °C (104 °F), but sometimes it can exceed 45 °C (113 °F). In return, in April and October, the maximum temperature is around 31/33 °C (88/92 °F), and sunshine is still guaranteed.
Average temperatures - Eilat
EilatJanFebMarAprMayJunJulAugSepOctNovDec
EilatJFMAMJJASOND
Min (°C)101215182225272725221612
Max (°C)212326313639404037332823
Min (°F)505459647277818177726154
Max (°F)707379889710210410499918273

In Eilat, the rainfall is very poor, since it doesn't even reach 50 millimetres (2 inches) per year.
Average precipitation - Eilat
EilatJanFebMarAprMayJunJulAugSepOctNovDecYear
EilatJFMAMJJASONDY
Prec. (mm)43321000042522
Prec.(in)0.20.10.10.1000000.20.10.20.9
Days2211100001129

In Eilat the sun shines regularly throughout the year, but occasionally some cloud banks can pass, even in summer.
Sunshine - Eilat
EilatJanFebMarAprMayJunJulAugSepOctNovDec
EilatJFMAMJJASOND
Sun (hours)78891011111110987

At Eilat, the Red Sea is a bit cool for swimming in winter, while it is pleasantly warm in the summer months.
Sea temperature - Eilat
EilatJanFebMarAprMayJunJulAugSepOctNovDec
EilatJFMAMJJASOND
Sea (°C)222121232526282828272523
Sea (°F)727070737779828282817773

When to go

The best times to visit Israel are spring and autumn, and in particular the months of April, May and October. Easter is therefore usually a good time to visit the sites of historical and religious interest in the country.
For a beach holiday, on the Mediterranean coast the warm and sunny season is long and runs from mid-May to mid-October, although as we have seen in May the sea is still a bit cool.
The Red Sea is warmer, but as mentioned in summer the heat is too intense for sunbathing, especially from June to September, while in winter, from December to February, pleasant days alternate with others which can be bit cold: so here the best periods for a beach holiday are the month of April, and from mid-October to mid-November.

What to pack

In winter: in Jerusalem and inland hills: warm clothes, jacket, hat, raincoat or umbrella, scarf for the wind, hiking shoes; in the Jordan Valley and Eilat: spring/autumn clothes, shirt for the day, jacket and sweater, umbrella for the area north of the Dead Sea. In Tel Aviv and the Mediterranean coast: spring/autumn clothes, sweater and jacket, raincoat or umbrella.

In summer: in Jerusalem and inland hills: light clothes, sun hat, sweatshirt for the evening, scarf for the wind-borne sand and dust, hiking shoes. In the Jordan valley and Eilat: lightweight clothing of natural fibres, desert turban; in Tel Aviv and the Mediterranean coast, light clothing, a light sweatshirt for the evening.
For the reef, snorkeling equipment, water shoes or rubber soled shoes.