What is the best month to travel in Europe?
Each time of year has its ups and downs and it depends on what your priorities are.
January-March: Colder weather and it can be rainy and gross (I feel like it rained the entire month of February in Brussels), but this is also when you can get better prices on hotels and tours because there’s less demand.
April-July: Generally better weather (although some places farther south can get quite warm) but also more crowded and more expensive. Honestly unless there’s a compelling reason to travel in these months (like a specific event, convention, show, etc.), I’d avoid them and leave them to the families with kids.
August: This is a big holiday month for actual Europeans and some cities (Brussels being a prominent example) will be ghost towns. The weather may be warm (and remember that a lot of places here don’t have air conditioning), but prices and crowds can be lower because everyone’s out of town — but this also means you don’t get the “lively” experience you would at other times of the year.
September-October: This is probably when I’d suggest you go somewhere. Summer tourist season has died down but the weather hasn’t started to get cold and wintry yet. You’re not paying rock-bottom winter prices but you can probably do better than you would in spring/summer. Just know that the top-tier cities (London, Paris, Rome, etc.) will probably still be busy.
November/December: The weather then isn’t the best but it’s also Christmas season and most major cities will have big Christmas markets and displays on in the last part of November. You may not get a price cut like you would in January/February but there is something dazzling about the Christmas market atmosphere.