Ribe's tourist season runs from April to October, with the peak in July and August when Danish families are on holiday and the day-trip coaches from Copenhagen and Hamburg are at their busiest. June and early September are arguably the sweetest months: long daylight, milder weather, fewer crowds.
Spring (March–April) and autumn (September–October) are the seasons of the Sort Sol starling murmurations, the single most impressive natural spectacle on this coast. Many visitors come specifically to time a trip around them.
Winter (November–February) is dark, wet and quiet. The cathedral, the museums and the inn restaurants stay open, but most other tourist operations close. Christmas in Ribe is genuinely lovely — a small old-fashioned Christmas market on Torvet, the cathedral lit at night, snow occasional but possible. The town is, if anything, more characterful in winter than in summer.