Because Mt. Gassan is the highest of the three sacred mountains of Dewa, it records the heaviest snowfalls. As hiking the mountain when there is still too much snow is dangerous, Gassan's pilgrimage season starts on July 1st, almost one month after Mt. Yudono.
The opening of the pilgrimage season in Gassan is called Mitobiraki 御戸開き ("the opening of the shrine's doors") and the end of the season, on September 15th, is called Mitojime 御戸閉め ("the closing of the shrine's doors"). The mountain is still hikeable until late September (if it's a warm year, until early October), but all religious activities are stopped, and all shops, facilities, and lodges are closed. We advise the inexperienced hikers/pilgrims to visit Mount Gassan during the pilgrimage season (July 1st to September 15th), so you always find water, food, help, and a place to stay in case of emergency.