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 Asāḷhi (roughly July); and the second, starting the day after the following full moon. At present, the first Rains-residence starts on the full moon in July, or the second if there are two. Why the Buddha formulated two periods of Rains-residence, the Canon does not say. From the Commentary's discussion of Mv.II.21.4, it would appear that if one enters the first Rains and then, for one reason or another, "breaks" the Rains (see below) within the first month, one would still be eligible to enter the second Rains so as to receive the privileges contingent on its successful completion.  In the Buddha's time, the determination of the lunar calendar was one of the responsibilities of the government in each kingdom or republic. Thus, to avoid controversy, the Buddha allowed that the wishes of kings be respected in this matter: If a king wanted to postpone the designation of the Asāḷhi full moon another month, bhikkhus were allowed to comply. (The rule coming from this origin story is stated in more general terms — "I allow that kings be complied with" — showing the general principle that we noted under Chapter 7, that the Buddha was not so foolish as to try to legislate for kings. The Commentary notes, however, that this principle applies only in matters in which the king's wish is in line with the Dhamma. No one, it says, should be complied with in matters where their wishes are not in line with the Dhamma.) At present, the governments of only a few countries concern themselves with calculating the lunar calendar for the sake of the general population. In other countries this point is not an issue, and the bhikkhus are free to calculate the lunar calendar without regard to the government's calculations.  

Entering for the Rains.  

The first day of the Rains-residence is when lodgings in a monastery are assigned for the duration of the Rains, so the Commentary recommends that a bhikkhu planning to spend the Rains in another monastery should start heading there a month before the start of the Rains so as not to inconvenience the assigner of lodgings and other bhikkhus there. As for bhikkhus planning to stay on in the monastery where they are already residing, they should spend the month before the beginning of the Rains preparing any worn-down buildings so that those who come for the Rains will study and/or practice meditation in comfort. 

The assigner of lodgings should assign lodgings for the Rains at dawn of the day the Rains begins. If other bhikkhus come later in the day and there are no extra spaces for them, they should be told that the lodgings have been assigned and that they should go to other lodgings, such as the foot of a tree. (What this means, apparently, is that they should enter the second Rains somewhere else, as the Canon contains a rule against entering the Rains in anything less than a proper dwelling. See below.)  

Mv.III.4.2 states that on a day for beginning the Rains one should not pass by a residence/monastery not desiring to enter for the Rains. How this applies to the beginning of the second Rains period is obvious: A bhikkhu must stop for the Rains on that day. As for the beginning of the first Rains period, the Commentary notes simply that if there are obstacles (see below), one may choose to enter the second Rains period instead. One obstacle not mentioned in the list below, however, is discussed in Mv.II.21.4. 

This is the case of a monastery where many (i.e., four or more) bhikkhus — "inexperienced and incompetent" — are staying for the Rains and none of them knows the uposatha or the uposatha transaction, the Pāṭimokkha or the recital of the Pāṭimokkha. One of them should be sent to a neighboring monastery immediately to master the Pāṭimokkha in brief or in its full extent. If he can manage it immediately, well and good. If not, one of them should be sent to a neighboring monastery for a period of seven days to master the Pāṭimokkha in brief or in its full extent. If he can manage it within seven days, well and good. If not, then all the bhikkhus should go stay for the Rains in the neighboring monastery. If they stay where they are, they all incur a dukkaṭa. None of the texts discuss the point, but apparently "going to stay for the Rains" in the neighboring monastery means entering the second Rains there. "Venerable Thanissaro Bhikkhu"  Read more>>