May to June and September to October are the best months. The Pyrenees are green, the trails are open, and the crowds of July and August are absent. Late May can still see snow on the high passes, but the lower valleys and Aínsa itself are warm and walkable.
July and August are the peak season. The old town fills with Spanish and French tourists; the Ordesa valley requires the shuttle bus and advance planning for parking. Aínsa itself is not unpleasant in summer — the old town stays cool in the evenings — but expect competition for tables and rooms.
November to March is quiet. Aínsa has a year-round population and the restaurants stay open, but many of the surrounding villages and mountain roads operate on reduced schedules. Snow above 1,500 m is common from December. The Añisclo canyon is walkable in mild winters; the high routes into Ordesa are not.
The Semana Santa (Easter) week brings a surge of Spanish visitors. The Sobrarbe Pyrenaica fair in Aínsa, usually held in late summer, celebrates mountain culture and traditional skills.