Unlike the Caribbean or Mediterranean, which sail year-round, Northern Europe's Norwegian Fjords and Baltic capitals itineraries are only offered roughly May through September. Outside that window, cruise lines reposition their ships to warmer regions rather than sail through the region's short, dark, stormy winter.
Within the season, June through August is the peak stretch — the warmest weather and, in Norway, the near-midnight-sun daylight the region is famous for. May and September are cooler, cheaper shoulder-season options with noticeably shorter days.
Because the season is so compact, popular sailing dates (especially July and August departures) book up earlier relative to the calendar than a year-round region like the Caribbean.