The season for walking the GR20 is fairly short. In-fact, the refuges stay open all year round but the guardians are only present, from the end of May, through to the end of September.
If you want to do the hike without having to use special alpine equipment, crampons, rope and ice axe, the trail becomes accessible in early June.
There are a lot of micro-climates in Corsica. No matter what period you choose to hike the GR20, you must be ready to confront wide ranging weather conditions. It is not unusual to come up against a host of different temperatures in the same day (thunderstorms, cold, fog, heat and sunshine.)
Whatever the season, bring the appropriate equipment and clothing. The weather can change quickly and the nights can be cold.