When planning a European trip, check your arrival and departure dates and see if they take into account peak, outbound and shoulder tours.
Peak areas are the times when most travelers fly to and from countries, and those times usually coincide with school holidays, Christmas, Easter and New Year. The country you fly will also have unique times for that country, so expect flights and shelters to be heavily booked and tickets will be more expensive. This peak period generally covers the end of December-January (due to the Christmas holiday) and the end of June-August (summer break). Traveling around Christmas can be difficult as plane ticket prices go up and ski resorts start to overflow. Public transport is also cut in many areas, as only a few trains leave on Christmas Day. The "shoulder season" is in the spring and fall, and the low or "off-season" periods are usually the remaining months and winters.
If you have limited time to see Europe, your best bet is to fly up and out. The prices are almost lower, and the travel conditions are excellent as you can avoid crowds and overestimates. If you have a lot of time to spend in Europe, try flying in the low season when the fares are cheap, so you have more money to spend on your leisure time. It is always a good idea to leave 3-4 months in advance to start checking your airline tickets. Airlines are starting discounts to reward early books and there is plenty of time to wait if you think they are too high. The cheapest ticket prices can appear at any time on the net or in your ticket office. And while you are at it, make sure you have booked your place 6 months before 2 weeks before leaving. It's a good idea to book a hotel for your arrival and departure days. The rest can be booked after that. Also remember to check for travel hazards and updates. Your government and insurance company can draw the line if you visit a particular area that has been warned you about. Keep up-to-date on the dangers and riots in the country you visit and make last-minute updates to check conditions immediately before departure.
For most seasoned travelers, the best time to visit Europe is from April to June and September to October, when the weather is still warm, the holiday crowds are gone, and accommodation and transport are regularly increasing.
In contrast, only the worst time to visit many parts of Europe (especially western and southern countries) is August. This is when many Europeans leave and spend their vacations in neighboring countries as well. Cities can be wilderness, and as a result residential prices are firing up and finding a bed can be very difficult. Buses and trains will often be replenished, and crossing the country can be a struggle to get a place. Even most tourist shops can be closed because the owners have gone on vacation. There is little doubt that July and August are Europe's busiest months (mainly because of Americans on vacation), and it will take a lot of patience and money. Avoid these months as much as possible. If you really need to travel in high season, your best practice is to stay away from big cities and choose off-road cities and villages and indulge in local culture.