Buddhist Lent Day or Khao Phansa Day marks the beginning of a period of three lunar months during the rainy season when monks are required to remain in one particular place or wat (temple). The tradition of Buddhist Lent or the annual three-month rains retreat is known in Thai as "Phansa". Khao Phansa is on the first day after the full moon of the eighth lunar month and ends on the full moon of the eleventh lunar month, or Ok Phansa Day.

The day before Khao Phunsa Day or the full moon of the eighth lunar month is "Asalha Puja Day". This day is very important in Buddhism. It was on this day that the Lord Buddha preached His Sermon to followers after attaining enlightenment.