When planning your European trip, check your arrival and departures dates and see if they take into account peak, off-peak and shoulder travel periods.
Peak periods are the times when majority of travelers fly in and out of countries and these periods usually coincide with school holidays, Christmas, Easter and New Year. The country you are flying into will also have peak periods unique to that country, so expect flights and accommodation to be heavily booked and tickets to be more expensive. This peak period generally covers late December to January (because of the Christmas season) and late June to the end of August (summer break). Travel during the Christmas holidays can be difficult as plane ticket prices soar and ski resorts start overcharging. Public transport is also reduced in most areas since only a few trains will leave on Christmas day. The "shoulder period" is in spring and autumn and the low or "off-peak" periods are usually in the remaining months and in the winter.
If you have only a limited time to see Europe, your best bet is to fly in and out during a shoulder period. Prices are almost at their lowest and travel conditions are great because you can avoid the crowds and the overpricing. If you have a lot of time to spare in Europe, try flying in the low season when fares are cheapest so you have more money to spread over your stay. It's always a good idea to start checking airfares 3-4 months before you leave. Airlines are starting to discount in order to reward early bookers and there's a lot time to wait if you think they're too high. The cheapest ticket prices might turn up on the net or at your ticket office at anytime. And while you're at it, make sure you've already reserved your accommodation 6 weeks to 2 months before you leave. It's a good idea to reserve a hotel for your arrival and departure days. The rest can be reserved afterwards. Also, remember to check for travel dangers and updates. Your government and insurance company might draw the line if you visit a certain territory they've warned you about. Be informed about the dangers and uprisings in the country you are visiting and do last-minute updates to check conditions right before you leave.
For most seasoned travelers, the best time to see Europe is from April to June and September to October when the weather is still warm, the holiday crowds have left and accommodation and transport are regularly priced.
In contrast, the single worst time to visit most parts of Europe (especially the western and southern countries) is the month of August. This is when many Europeans leave and take their holidays in nearby countries as well. The cities can be deserted and as a result, accommodation prices shoot up and finding a bed can be very difficult. Buses and trains will often be packed and it can be a struggle to get a seat while you cross countries. Even most tourist shops and sites can be closed because the proprietors have gone on holiday as well! There's little doubt that July and August are the busiest tourist months in Europe (mainly because of Americans on vacation) and it will require a lot of patience and money. Avoid these months as much as you can. If you really must travel in the high season, your best course of action is to stay away from the major cities and choose the off-track towns and villages and indulge in the local culture.