Seyðisfjörður is most visited in summer (June to August), when the fjord is green, the waterfalls are at their peak, and the LungA festival (late June or early July) brings a burst of creative energy to the harbour. The midnight sun means you can walk around the fjord at 11pm in golden light. This is also when the Smyril Line ferry runs most frequently, so the town fills briefly each week as passengers disembark and explore before moving on.
The ferry also runs a reduced winter schedule, and arriving in winter requires some planning but rewards it with a genuinely remote feeling, dramatic low light, and occasional northern lights over the fjord walls. The road from Egilsstaðir is sometimes closed by snow or ice — check conditions before driving, and be prepared for the pass to close overnight.
Spring (May) and autumn (September) offer the best balance of quiet and accessibility. The Fjarðará waterfalls are full in spring, and in September the surrounding hills turn rust and gold. Both seasons are good for photography in the sharp East Fjords light.