Ystad works in most seasons. Summer (June–August) is the obvious choice: the beaches are in use, the outdoor cafés fill Stortorget, and Ales Stenar is best appreciated in long evening light. The town gets a manageable influx of Scandinavian summer tourists but never becomes crowded. The ferry to Bornholm is busiest in July.
Spring and autumn are in many ways better. The medieval streets are quieter, the light on the half-timbered houses is lower and more atmospheric, and the Skåne farmland around town is either green with rapeseed or golden with harvest. Ale Stenar in October, with a Baltic wind and nobody else about, is one of the better experiences in southern Sweden.
Winter is quiet to the point of deserted in parts. The beaches are closed, some restaurants reduce hours, and the days are short. But the town itself is intact and picturesque under winter light, the museums stay open, and prices drop. The watchman still blows the horn from St Mary's tower every quarter-hour through the night.
Temperatures: summers are mild (18–23°C), rarely hot. The coast keeps winters from being severe (0–5°C average January). Snow is possible but not guaranteed.