Temperatures can vary widely, so just pack for the "right"
weather and you'll be fine. Seasons overlap of course, but
general guidelines are:
Spring (late Mar/Apr/May): Most of the country will have very pleasant
daytime temperatures of 60-70°F (15-25°C). Jerusalem will be
cooler in the evening.
Summer (Jun/Jul/Aug/early Sep): Tel Aviv and Tiberias will be hot and
humid with temperatures in the high 80's (30°C) and 90's (35°C).
Jerusalem is dryer and cooler, particularly at night. Masada and
Eilat are extremely hot - 110°F (45°C) but dry.
Autumn (late Sep/Oct/Nov): As the spring.
(Dec/Jan/Feb/early Mar): Winter weather can fluctuate between
mild and sunny, or severe and overcast. Expect northern
temperatures of up to 60°F (15°C) and 40°F (10°C) in Jerusalem
and the Galilee hills, becoming very cold at night. Eilat and the
Dead Sea remain hot.