The GR20 is a very popular hike with a relatively short season and is estimated that over 10.000 hikers hike the trail every year. Due to limited spatial availability in most of the refuges (most of them have a capacity of roughly 35 indoor sleeping options), it is advised that you book your tour early as accommodations along the route tend to fill up quickly. We would recommend you book the tour a couple of months in advance, preferably before winter if you are planning your hike in the following year.