Weimar is a year-round destination, but spring and early autumn are the sweet spot. May through June and September through October bring mild weather, manageable crowds, and the Park an der Ilm at its best. Summer (July-August) is warm and busy with tour groups, particularly around the Goethe and Schiller sites.
Winter can be cold and grey — Thuringia sits in the continental heart of Germany — but the museums are less crowded and the city takes on a quieter character that suits it well. The Weimar Christmas market on the Markt is smaller than the famous ones in Dresden or Nuremberg, but genuine and pleasant.
The Kunstfest Weimar, held annually in late August and September, is the city's premier cultural festival, with theatre, music, and visual arts performances across multiple venues. The Weimarer Fruhjahrstage (Spring Days) in March features classical music. If your trip coincides with either, book accommodation well in advance.