Wolfenbüttel is a year-round destination, though summer (June–August) is the most comfortable time to walk the old town streets, and the Herzog August Bibliothek runs its most substantial exhibition programme then. Spring is excellent — the Oker riverside is green and quiet, and crowds are thin compared to larger cities.
Winter brings its own atmosphere. The half-timbered facades look especially handsome under snow, and the town runs a modest Christmas market on the Stadtmarkt in December. The cold months are also the best time to spend hours in the library's reading rooms without feeling guilty about not being outside.
The annual Wolfenbütteler Kultursommer brings outdoor concerts and events in summer. The library hosts its own lecture and reading series throughout the year — worth checking the programme before you visit.