Prague has a continental climate and sees all the four seasons. Winters are very cold and most years it snows, especially around January and February. Spring is mild and sunny but with plenty of rain showers so an umbrella is invaluable. Summers can get very hot so plenty of suncream and a hat will make things a little more pleasant. Late summer and autumn is a time of beautiful copper shades, thunderstorms and sun.
Each season has its pros and cons when it comes to sightseeing. For example a snowy winter Prague makes an ideal setting for the Christmas Market but requires warm boots and clothes and a sunny, hot summer requires less clothes which makes for a lighter suitcase. Either way, Prague has much to offer regardless of the weather but it is a good idea to keep in mind the season when packing your suitcase.