Freiburg is the warmest city in Germany. Summers are reliably hot — Mediterranean heat at the height of July and August, when the city bakes and the locals decamp to the Dreisam river. Winters are mild by German standards, with snow uncommon in town but reliable in the Black Forest above.
Late spring to early autumn (May to September) is the obvious choice. The Münsterplatz market is at its best, terraces are open, and the surrounding wine country is in full swing. The wine festival on the Münsterplatz in late June is the standout event, with two hundred Baden wines available by the glass.
Autumn (October to November) brings the new-wine festival, the chestnut trees on the Schlossberg turning gold, and the start of the game season in the restaurants. Winters are quiet, but the Christmas market on the Rathausplatz and the Kartoffelmarkt is one of the most attractive in southern Germany. If you are here in February, the Fastnacht (carnival) is celebrated more vigorously than almost anywhere else in Germany.