Visit Mykines in summer, roughly May to August, when boats run and the seabird colonies are active. Puffin viewing is usually best in the heart of the season, but access rules can change to protect nesting birds.
Even in July the weather is not gentle. Fog, wind, and swell can close the island for a day or more, so treat Mykines as a flexible plan rather than the fixed centrepiece of a short trip.