The highly important time a year for Theravada Buddhist monks is a three-month-long Vassana or ‘Rains Retreat’ period. It comes right after the full moon of Asalha month which is around July every year. The first Buddhist monks did not reside in monasteries, they travelled through villages offering their teaching, collecting alms, and sleeping beneath the trees in the forests. However, monsoons that struck South and Southeast Asia during that season, made travelling too complicated and dangerous. Eventually, monks and nuns started being invited to village monasteries and nunneries for the duration of rains.
Buddha himself started the tradition of the Rains Retreat in the year 588 BCE when the Lord along with 60 bhikkhus and his 54 friends resided in Sarnath vicinity near Varanasi. Therefore, the period of Lent or Rains Retreat means a lot for ordinary Thai people. At this time most monks are to stay at a single monastery without changing their locations and dedicating their time solely to religious aspects. During this time, many vendors will be slightly quiet, and the atmosphere will be a bit more solemn and spiritual.
For lay people, the Vassana traditionally used to be the period when one could come to resident monks and ask for Dhamma teachings or spiritual guidance. In the Theravada School of Buddhism, the unique tradition of observing the Vassana has survived to the present day.
The Lent starts depending on the lunar calendar, but typically in July on Khao Phansa Day, and finishes on Awk Phansa Day, typically in October.