Erfurt is pleasant year-round, but the two best times to visit are summer and the Christmas season. In summer the Domplatz and old town come alive with outdoor cafes and the DomStufen-Festspiele opera festival, which stages productions against the backdrop of the Cathedral steps in late August — a spectacular setting.
The Christmas Market, held on Domplatz from late November through Christmas Eve, is one of the most beautiful in Germany. The Cathedral illuminated behind the market stalls, the smell of mulled wine and Thuringian bratwurst, and the relative lack of international tourists compared to Nuremberg or Rothenburg make it an atmospheric alternative.
The coldest months (December–February) are manageable but grey. Spring, when the lindens flower and the parks fill with people, is lovely. Avoid the first weekend of the Bundesgartenschau years if you want to explore without crowds.