In the eighth lunar month, Preecha Phinthong cites two Buddhist holy days which should be observed--Asanhabucha Day (ອາສານຫະບູຊາ, the full moon day of the eight lunar month which marks the anniversary of the day on which the Buddha delivered the First Sermon to his first 1250 disciples at Deer Park in Benares) and Buddhist Lent (the day after Asanhabucha Day). Activities relating to these holy days include preparation of candles to offer to monasteries, offering monks' robes, incense sticks, candles, and oil to the monks to use in their meditation and study of Buddhist Scriptures during Lent, and listening to sermons.  Maha Sila Viravong mentions only the Buddhist Lent in his Heet Sipsong (The Twelve Monthly Ritual).    He also gives a story which was the origin of Buddhist Lent.

khao phansa Day
(eighth lunar month : making merit during the Buddhist Lent)

Every monk has to stay at his own monastery for three months during the rainy season in order
to study dharma and keep Precepts. The lay people offer alms, keep Precepts, and meditate.

During the rainy season, most traders would stop travelling because the paths were mostly muddy.  Yet, the Buddha's disciples did not stop their travelling.  The people began to complain about how those disciples trampled on their rice fields, spoiling the rice crops.  When the Buddha heard those complaints, he set up the rule that Buddhist monks must stay in their temple for three months beginning from the first waning moon of the eighth lunar month until the first waning moon of the eleventh lunar month.   During this time, the monks must not stay overnight anywhere else except for their own designated temple unless there were specific urgent matters specified in the Scripture.