Summer (June to August) is the best time to visit Tallinn — the days are long (midsummer barely gets dark), the Old Town's outdoor cafes and terraces are at their best, and the cultural calendar is packed with the Tallinn Music Week, the Old Town Days festival in June, and outdoor events throughout the season. The Tallinn Black Nights Film Festival (November) is one of the best in Eastern Europe. Winter transforms the Old Town Christmas market into one of the most atmospheric in Europe (December). Spring and autumn are mild and relatively uncrowded.
Avoid the Old Town in late July during school holidays if you dislike crowds — Tallinn has become a very popular cruise ship destination and the streets fill dramatically between 10am and 6pm. Ferry connections from Helsinki mean the city also gets day-trippers at volume on summer weekends.