Warsaw is a destination that you can travel to at anytime of the year. Summer is surprisingly warm, and you find that people tend to spend a lot of time wandering the parks relaxing. Winter on the other hand is very cold and dark, and the city can appear to be quite miserable, although in snow, the old town is very picturesque, and the festive spirit more than makes up for the chilly temperatures.
There are major events and public holidays in the city throughout the year, and consequently, no matter what time of year you visit Warsaw, the chances are that there will be at least something going on.