Špindlerův Mlýn is a proper two-season resort. The ski season runs from roughly December through March, though reliable snow is not guaranteed every year — the resort has snowmaking on the main pistes, which extends the season. January and February are the coldest and most reliably snowy months.
Summer (June–September) brings hikers, cyclists, and families who use the lift infrastructure to reach the high plateau without the climb. The wildflower meadows on the upper slopes are at their best in June and July. September is uncrowded and the autumn colours in the valleys are worth seeing.
Shoulder seasons (April–May and October–November) are quieter and cheaper, but many mountain lodges close and the high trails can be muddy or snow-patched. The resort village itself stays open year-round.