May - September is the best time to visit Eastern Europe
The climate of Eastern Europe is highly diverse ranging from mediterranian to alpine to arctic. The temperatures are warm during this time making it easier to travel. The region is filled with tourists enjoying the region's natural beauty and various festivals.
The temperatures are warm but not excruciatingly unbearable. The region is crowded with tourists during this time. Summer is a good time to vist Eastern Europe especially if you're planning on exploring it's natural beauty. However, hotel pricesmay be steep during this time.
The temperatures are mostly agreeable during this time. The weather could be slightly chilly at times so it's best to pack accordingly. The hotel prices are cheaper than they are in the summer months. It is not too crowded making it easier to explore the region.
The temperatures are quite low during this time and these are the coldest months of the year. The region is less crowded and hotel prices are low. People mostly visit during this time for the winter sports such as skiing, ice fishing, snow boarding etc.
The weather is pleasant during this time. Nature is at its best with the flowers in full bloom and it is a good time to observe the region's wildlife. Hotel prices are reasonable during this time. You may find light crowds at certain places but the region isn't overly crowded during this time.