Late July is the most dramatic time to visit Paderborn, when the nine-day Libori festival transforms the city. The old town becomes a carnival, accommodation fills up well in advance, and you will encounter up to a million visitors over the festival period. If you want this experience, plan ahead; if you want a quieter visit, avoid it.
The rest of the summer is pleasant and the town centre is lively with the usual German street-café culture. Spring and early autumn are the best times for walking in the park around the Pader springs. Winter is cold and grey; the Christmas market in the old town runs in December and is a moderate-sized affair.