Leipzig is a year-round city. Spring and autumn are the best times for a comfortable visit — mild temperatures, longer days (from April), and the city's cultural calendar in full swing. The Wave-Gotik-Treffen in late May/early June is either a delight or a reason to stay away, depending on your tastes.
Christmas is good: Leipzig's Weihnachtsmarkt is one of the most atmospheric in eastern Germany, set on the Markt from late November through December. Winter is cold but the museums are warm, and hotels are cheaper than in summer.
July and August can be hot. The trade fair calendar (Leipzig hosts major book, auto, and games fairs) affects hotel prices significantly — book ahead during the Leipzig Book Fair in March, which draws 250,000 visitors.