There are always peak travel times for every destination, but for the best rates on flights and hotels, plan around those and go in the off season instead. Europe's peak season is June through August, so going to Paris or Milan in late October while the weather is still somewhat nice is a great idea. Theme parks are always busy when school is out, so avoid Summer, Spring break, and Christmas (book a trip right after school starts!). And the tropical beaches of Hawaii or the Virgin Islands are busiest December through April, so going in late Spring or early Summer is ideal.