Bamberg works year-round. The best stretch is May through early October — beer-garden weather, long evenings on Sandstraße, and the city's two big festivals (the Sandkerwa folk festival on the last weekend of August and the Fischerstechen fishermen's joust the Sunday before). Summer brings serious heat sometimes but the river-cooled lower town is fine. September is harvest time in the surrounding villages and a good month for keller-hopping.
Winter is cold and dark but the city centre wears it well: Christmas market on Maximiliansplatz in December, occasional snow on the cathedral towers, and the half-timbered houses lit yellow against grey skies. Avoid carnival weekend in February if you do not like crowds.