Temperatures can vary widely so just pack for the "right" weather and you'll be fine. Seasons overlap of course, but general guidelines are:
SUMMER (Jun/Jul/Aug/early Sep): Temperatures in the high 80º's, 90º's. Tel Aviv, and Tiberias will be hot and humid (like New York or Miami). Jerusalem is dryer and cooler, particularly at night. Masada and Eilat are extremely hot (110º+!)...but dry. There won't be a drop of rain.
SHOULDERS (late Mar/Apr/May/late Sep/Oct/Nov): Daytime temperatures will be very pleasant: 75º-ish in most of the country (but hotter at the Red and Dead Seas). Jerusalem will be in the 70º's, 50º's in the evening. There'll be some rain - nothing torrential.
WINTER (Dec/Jan/Feb/early Mar): Winter weather can fluctuate. Some winters are mild and sunny, some severe and overcast. There's often heavy rain and, in January and February, even snow sometimes. It'll probably be in the 50º's, 60º's most places, but in Jerusalem and the Galilee in the forties, and cold at night.
