Jerusalem enjoys long, warm, dry summers (May-September) and cool winters (November-March). Often the seasons change rapidly from very cold to very hot, but you can normally experience the pleasant season of Spring for a brief period of time around the month of April and the mild season of Fall during October. At 809m above sea level, and very much within the Judean desert hills, Jerusalem is generally cooler and less humid than Tel Aviv.
Summer months are generally without rainfall with average daily temperatures between 20° and 30° Celsius (70°-90° Farenheit), although temperatures rise somewhat in the scorching months of July and August. During peak sunshine hours in the hottest months, or during a heat wave (sharav, or chamsin), you’ll want to remain indoors in an air-conditioned environment. When outdoors, precautions should be taken against sunburn and sunstroke, namely wearing a hat, sunglasses, sun cream and drinking plenty of water.
With regard to rain - you'll know when it won't rain, but not when it will. The rainy season is during the intermediate and winter months, from around October to April (from the Sukkot to Passover holidays, just like in the Bible). Winter can be extremely cool and wet, with rain, the occasional snow storm, and temperatures sometimes down to freezing. However, even during the winter, many days have some sunshine.
Click here for Jerusalem weather forecast.