Vassa (from Pali vasso, Sanscrit varsah) means Rain, also called Rains Retreat or Buddist Lent, is the traditional retreat during the rainy season lasting for three lunar months from 1 Roch of Arsath to 15 Koert of Aksuch (from July to October), originally last 4 months from 1 Roch of Arsath to 15 koert of Kadeuk (from July to November). During this time Buddist monks remain in a single place, generally in their temples. In some monasteries, monks dedicate the Vassa to intensive meditation practice, pray and contemplate (study). During Vassa, many Buddhist lay people reinvigorate their spiritual training and adopt more ascetic practices, such as giving up meat, alcohol, or smoking.
Buddhist lent is the most important religious holiday for monks, novices, and lay people. Monks stop wandering and stay on the temple grounds. Novices benefit because they receive additional education. Laypeople stop doing evil and cut back on bad habits. Some people take this opportunity to purify themselves and start a new life.
The origins of the vassa tradition are ascribed to early Buddhist times. Gautama Buddha ordered his disciples to observe a pre-existing practice whereby holy men avoided travelling for a three month period during the rainy season, in order to avoid damaging crops. There are two parts, the first part, for the entering of the rainy season, is called "Choul Phreah Vassa". The second part, for the exit of the rainy season, is called "Cheanh Phreah Vassa". While lay people are free to chose what level of discipline they wish to exert in following the rules of the lent. For monks, however, strict observance is mandatory.
A few days before the Buddhist Lent, Cambodian people buy pairs of big candles decorated with dragons, flowers or many kinds of artistic adornment on the surface. One candle is for the buyer and one to get a husband or wife in the next life. The day before Buddhist Lent, the 15th day of the dark moon, commemorates the first sermon of Buddha to his first five disciples. Crowds of people gather in the temple in order to listen to the Dharma.
On Buddhist Lent Day, Buddhist worshippers go to the temple, offer food and others necessities to the monks, and attend a 3-round candle procession around the Buddhist temple with the lighted candles. During this time a senior monk (one who has been a monk for longest) announces that the first day of the there months of Bonn Choul Preah Vossa. During this period, we are not allowed to go anywhere and all monks who are seated with him repeat the pledge after him. Except urgent case such as closed relative (mother or father) fails into very sick, they can leave the pagoda with abbot's permission of maximum of 7 days.
The end of vassa (The end of the retreat period observed during three months, October) is marked by joyous celebration. The following month, the Kathina ceremony is held, during which the laity gathers to make formal offerings of robe cloth and other requisites to the Sangha.
Source: Internet