The winter months of November to March are the coldest in Israel with snow in the northern reaches of the Golan Heights, and occasional snow fall in Jerusalem. The coast experiences the heaviest rainfall at this time though temperatures in places like Eilat are reasonably pleasant still.
Jerusalem and other highland areas are splendid in the summer (June - August) when temperatures are high with balmy evenings, and little chance of rain. Tel Aviv and the far south of the country are usually several degrees warmer, making it the perfect place to soak up the sun beach side. If you plan on visiting the Dead Sea or Negev Desert be sure to pack a hat to protect you from the sun.
The seasons of spring (April and May) and autumn (September and October) are the best time to visit Israel when temperatures are pleasantly mild throughout the country. During these spring and autumn months pack clothes that you can wear in layers, so that whether sunny skies or cool weather you will always be prepared.