Late spring through early autumn is the best window for Wrocław. May and June bring long evenings and the cherry blossom in the Japanese Garden; July and August are warm and busy with students gone and tourists arriving. September is perhaps the best month — still warm, lighter crowds, and the start of Wratislavia Cantans and several other major festivals.
Winter is cold (often below zero from December to February) and the days are short, but the Christmas market on the Rynek is one of the best in Poland, with mulled wine, grilled sausages, gingerbread and craft stalls running for the whole month of December.
Wrocław is a serious festival city. Wratislavia Cantans (September) is the biggest classical event; the Wrocław Non-Fiction Festival in May, the New Horizons film festival in July-August, and Brave Festival in summer all draw international audiences.