We had a pretty good summer but some days were really too hot. At least for my liking....the temperature can reach up to 35 degrees of Celcius and that is just way to hot for sightseeing. Also the downfall of traveling in summer is much higher number of tourists pouring into the city. So like I said, great months to come would be September/October and then May/June. Winters can get really cold so it might not be the best time to visit neither unless you like to come for skiing or winter sports but to be honest for that Austrian Alps are much better... I am sorry to say that... If you do want to come in winter though, than December is a great month due to Christmas Markets in Prague that were voted between the top 10 Christmas markets in the world.