May through September is the obvious window, when the Rhine and Moselle steamers run their full schedules, the cable car keeps long hours, and the riverside terraces are open. June and August can be hot; September is reliably warm with smaller crowds and the start of the wine harvest in the gorge.
The second weekend in August is Rhein in Flammen — a synchronised fireworks display along the gorge, with the highlight at Koblenz itself. The hotels are booked a year ahead. The Christmas market on the Jesuitenplatz runs through late November and December and is more atmospheric than the bigger ones in Cologne or Frankfurt.