Month by month
Late May–June: valleys and mid-altitude trails are excellent; high passes often still hold snow and many huts open only from mid-to-late June.
July–August: everything is open, days are long, and huts and popular trailheads are at their busiest. Afternoon thunderstorms are common — start early.
September: stable weather, clear air and thinner crowds. Huts start closing from mid-month, so check dates before planning a hut-to-hut route.
Altitude matters more than the calendar
A 1,500 m valley trail can be walkable in May while a 2,800 m pass two hours away is not. Guides plan the altitude to the date rather than the other way round.
Related questions
Can you hike in the Alps in winter?
Yes, on prepared winter walking trails and snowshoe routes. It is a different discipline with avalanche considerations, and a qualified winter leader or guide is strongly recommended.