Krka National Park in January

May hits the sweet spot - warm enough for swimming (barely), small crowds, everything green from spring rains. The waterfalls run strongest after winter snowmelt, creating more dramatic cascades than you'll see in late summer. June through August brings perfect swimming weather but also brings everyone else. Arrive before 9 AM or after 4 PM to avoid the worst crowds. The swimming areas become literal human soup by midday in July. September and October offer the best compromise. Water's still warm from summer heating, crowds thin out after school starts, and autumn light makes everything look golden. Some boat services to Visovac reduce frequency after September 15th. Winter visits work for serious hikers who don't mind skipping the swimming. Entry fees drop significantly, and you'll have the trails mostly to yourself. But many facilities close, and weather can turn quickly - check conditions before driving from Split or Zagreb.