With so many cultures to show off — and so many centuries of practice — Europe is a pro at partying. The normal can-can of carnivals, medieval fairs, music fests, and national holidays could light a fire under the most burned-out sightseer.
Keep an eye out for major events that coincide with your itinerary — even festivals that don't interest you are worth noting. For example, you're headed to Munich in early fall, be sure to check this year's Oktoberfest dates, whether or not that's your idea of fun. If it is, it'd be a shame to miss it by assuming the beer was still flowing in mid-October (despite its name, most of the party takes place in September). If tent-fulls of drunken Bavarians are something you'd rather avoid, it'd be a shame to plan a September visit and unexpectedly encounter fully booked hotels, higher room rates, and a crowded city center.
Remember that event dates can change, so be sure to verify dates on each festival's website or with a local tourist information office.