It's Known as Chol Vossa or 'Buddhist Lent', this day marks the beginning of the three-lunar-month period, during which all monks should stay in their temples and not travel without special reasons.

Bebefore the Buddha's time there was a custom in India that wanderers would stay in place for the rainy season, both to avoid having to negotiate muddy roads and to avoid trampling plants. Bhikkhus in the early years of the Buddha's teaching career were criticized by the Jains for not observing this custom, so the Buddha gave his permission for them to stop their wandering for three months of the Rains. Later he imposed a penalty for not observing this custom. 

Periods of residence.  

Because the rainy season in South Asia is roughly four months, bhikkhus are allowed to choose between two periods of Rains-residence: the first, starting the day after the full moon of the month of…