There is no right or wrong time to travel through Europe. The summer months, peak season, tend to see the nicest weather, but also the biggest crowds and the highest prices. Some cities including Dubrovnik, Barcelona and Venice have been experiencing severe problems with overtourism during the peak season, which can naturally have an impact on your enjoyment. But other parts of Europe, the Baltics, Scandinavia or the UK for instance, are best seen in peak season when the weather is warmest and most reliable.
Conversely, the off-peak season, which normally lasts between November and March, involves much more unpredictable weather, with rain, snow and cold temperatures likely in many parts of Europe, and shorter days. But at the same time, visitor numbers drop off sharply in most places, slashing queues at even the most popular landmarks.
The shoulder seasons of late spring and early autumn are a good time to visit Europe. Through April, May and early June, and late September through to October and November, you should enjoy smaller crowds, but still pleasant weather, and plenty of daylight. The countryside is normally at its most beautiful in these months as well.