The best time to visit St Kilda is generally from late spring to early autumn, specifically from May to September, when the weather is at its most favourable and wildlife is abundant.
During these months, you'll experience longer daylight hours, which are great for exploring the islands and wildlife watching. The sea conditions are typically calmer, making boat trips to and from St Kilda more reliable. May and June are particularly good for seeing the vast seabird colonies, as this is their breeding season. You'll witness thousands of puffins, gannets, and fulmars nesting on the cliffs.
For those interested in hiking and enjoying the dramatic landscapes, July and August offer the warmest temperatures, though you should always be prepared for changeable weather. This period is also ideal for spotting marine life like whales and dolphins around the archipelago.
Later in the season, September can still offer pleasant conditions with fewer visitors, providing a more tranquil experience.