Depending on where you are in Israel and the time of year, 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 80-'s, 90-'s (26-32c.). Tel Aviv, and Tiberius will be hot and humid (like New York or Miami). Jerusalem is dryer and cooler, particularly at night. Masada and Eilat are extremely hot (110+!)(43c.)...but dry. There won't be a drop of rain.
SPRING/FALL The best for a tour to Israel. (late Mar/Apr/May & late Sep/Oct/Nov):
Daytime temperatures will be very pleasant: 60-70-ish (15-20c.) in most of the country (still hot, though, at the Red and Dead Seas). Jerusalem will be in the 40-50-'s (5-10c.)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 50-'s, 60-'s (10-15c.) most places, but in the 40-'s (5c.) in Jerusalem and the Galilee hills - and very cold at night. A tour to Israel in the winter is possible and with some luck - can be very enjoyable.