Following the full moon day in July is the beginning of Buddhist lent, which the Buddha assigned to all of his disciples. It is said that Buddhist monks traveled in every season, especially in the rainy season, when sometimes they accidentally stepped on young plants and insects. People complained about this matter and so the Buddha called the monks to a gathering and said, “Behold monks, I grant you all to stay in the monastery in the rainy season.”
The commentary says that there are two forms of the Buddhist lent; 1. The first one is directly after the full moon day of July. 2. The second one is after the new moon day or a month after the original day.
The regulation is that all monks have to stay in the temple for three months, they are not supposed to over night anywhere during this time. If, for some unavoidable reason they have to travel, they must catch dawn in the temple. In the case of extreme emergencies or absolutely necessary journeys, they may be away for seven days, before their return.
There is an ancient tradition that the monks repeat three times saying “I stay in this monastery for three months” this is the Buddhist lent as proclaimed by the Buddha.