The Kathina festival, aka “Buddhist Lent”, is a major observance of Theravada Buddhism. It is a time for laypeople to offer cloth for robes and other necessities to the monastic sangha. Kathina takes place every year in the four weeks following the end of Vassavasa, the rains retreat.

The three months prior, Vassavasa, The Rains Retreat : During the rainy season the Jain (spiritual practice well before the Buddha’s time) mendicants developed the notion of non-injury (ahimsa) and paused their wandering habits. Respecting this as well, the Buddha ordered his followers to pass the rainy season in settled dwellings.

What began simply as a 3-month pause in the practice routines of nomadic monks became an institutional development that would greatly influence lay and monastic cooperation.

Often, the rains-retreat practice is not limited within the Buddhist Saṅgha, but lay followers can be found observing the period. During “Buddhist Lent”, lay followers take certain vows and observe rigorously, engaging in activities such as: providing alms food to monastic Sangha, giving up smoking and intoxicants, observing the eight precepts, practicing meditation, chanting Buddhist Suttas and listening to Dhamma talks. At the end of the rains-retreat the Buddhist Sangha performs a special ceremony called the pavaraṇa, where each one invites his fellows to point out any mistakes committed during the retreat period. It is well documented in the texts that during the rains-retreat, many monks and nuns achieved spiritual goals. The rains-retreat affirms that both monastic and lay community have developed over time in mutual co-operation and observance of the Buddha’s teachings.

Undoubtedly, with the institutionalization of the rains-retreat, the Buddhist Sangha has performed an important role in the development of Buddhism.

Information from Buddhistdoor.net