Czech Republic has a continental climate with cold winters and hot summers. Please, not that weather in Prague is very changeable and a nice sunny day can turn wet with little or no advance notice. Carrying an umbrella with you may be a good idea.
The weather is in its best from May though late September. If you plan to travel in mid-winter be ware of a possibility of smog caused by coal heating in most of the Prague houses. In July and August the city is very crowded with tourists. Many think that June is the best time to visit Prague but personally, I like it around Christmastime for its transparent airy look, Christmas bazaars and children singing carols on the city squares.
Today weather:
