What is "open season" (kaizan-ki)?

Kaizan-ki〈開山期〉literally means "the period of time when the mountain is open" and typically shorten as "open season". In case of Mt. Fuji, it is officially defined by local stakeholders and municipalities, conventionally corresponding to early July–early September, during which public amenities along trails on Mt. Fuji, including mountain huts, toilets, and other facilities and services operate above the 5th Station of each major route, and visitors heading for the summit are publicly accepted and encouraged.

The opposite of kaizan-ki is referred to as heizan-ki〈閉山期〉, or "closed season" when Mt. Fuji becomes just as wild as other remote mountains. Note it is not that entering to the upper part of Mt. Fuji is strictly prohibited during this period although it is not reccommended to attempt climbing off-season as conditions become considerably riskier.