Summer is high-season across Europe. High-season brings the most crowds, the highest prices and the lowest availability. Unless you are visiting Scandinavia, and have no desire to freeze your ass off in the winter months, I would not recommend planning your Europe trip in the middle of the summer. Shoulder season (April, May, June and September) tends to be the best time to visit Europe. However, there are exceptions to this rule. For example, if you're visiting Prague – which has no beaches and not many famous summer activities – you may consider going in February, March, October or November for even lower prices and less crowds.