In the early days of the Buddha's ministry, he did not enact any rules for the monks, making it compulsory for them to take up rains residence. The rainy season in India (around June to October) was the time when crops are sprouting everywhere. When the monks wander around during the rainy season, they trampled over the young plants. The farmers were naturally unhappy about this. The matter was brought before the Buddha whom then laid down the rule which makes it compulsory for the monks to take up the rain residence in fix abode for a period of 3 months during the rainy season.
During this 3 months, the monks must not leave the place where he has "enter vassa". If he have to leave (for a valid reason), he must return within 7 days or he will break the vassa. The number of vassa he has sucessfully observed reflects the seniority of monks.
For the monks, the 3 months of Vassa is the time devoted for either intensive meditation or studies of the Dhamma. To the lay devotees this is an opportune time to make merits by supporting the monks with their material needs for the whole period of the vassa. It is also a good time for them to meet the monks for dhamma discussion.
Just before the vassa begins, the lay devotees would offer the monks robes known as rain robes together with other requisites. The merits gained from offering rain robes is capable of generating other new merits. This happens when the lay devotees, after offering the robes, start to think of next year offering. Or they think of the monks during the vassa period, who is using the robes they offer and derive happiness from that thought. All these wholesome thoughts after the offering, generates fresh wholesome merits for the devotees.
Extracted from VMC newsletter