Aachen is a year-round destination, but the city has a distinct high point: the Christmas market in December, one of Germany's most atmospheric, spreading from the Rathaus steps across Am Markt to the base of the Dom. The warm yellow light of the stalls against the medieval stonework is genuinely beautiful, and the Glühwein and Printen are the real thing.
Summer is pleasant — the proximity to Belgium and the Netherlands means milder temperatures than further east in Germany — and the outdoor cafe season runs from May through September. The CHIO Aachen equestrian tournament in late June/early July is one of the most prestigious in the world and draws visitors from across Europe. The university calendar keeps the city lively from October through June; it goes noticeably quieter in August when students leave.