The day following Asanha Bucha in Thailand is called Wan Khao Phansa. It’s the first day of the Thai Phansa (พรรษา), the annual rains retreat observed by Theravada practitioners. Taking place during the wet season, Phansa (also known as Vassa, or Buddhist Lent) lasts for three lunar months, usually from July to October (from July 20 to October 16 in 2016). For the duration of this period, monks remain inside monasteries practicing intense meditation. At the same time other adherents to the Buddhist faith refrain from eating meat, alcohol and smoking tobacco.
The practice of remaining within the temple grounds originated when the Buddha ordered his disciples to observe a pre-existing practice during the rainy season in which holy men avoided travel for a three-month period in order to avoid damaging crops growing in the fields. Another significance is attached to this festival, wherein, Buddha’s son Rahul was said to have been born during this period. It was after his birth that the Buddha gave up all his Royal worldly trappings and set out to attain spiritual enlightenment and the meaning of life.
The number of years a monk has spent in monastic life is counted by the number of phansas he has observed. Phansa begins on the first day of the waning moon of the eighth lunar month (the day after Asanha Bucha or Dharma Day) and ends on Wan Ok Phansa (วันออกพรรษา) when all monks come before the community of monks and atone for any offences they may have committed during the rainy season.
As far as I know, Thailand has never issued a stamp directly celebrating Wan Khao Phansa. There have, of course, been numerous releases over the years with Buddhist themes. Scott #323 was part of a set of nine issued on May 13, 1957, marking the 2500th anniversary of the Buddha’s birth. This stamp features the dharmacharka (literally, ‘wheel of Law’), a Buddhist emblem resembling a wagon wheel — the circle symbolizing the completeness of the Dharma —with eight spokes, each representing one of the eight tenets of Buddhist beliefs. The spokes represent the eightfold path leading to enlightenment: Right faith, right intention, right speech, right action, right livelihood, right endeavor, right mindfulness, and right meditation. Sometimes, as pictured on this stamp, the dharmachakra is flanked by deer referring to the deer park in which the Buddha is said to have given his first sermon. The bright green Scott #323, denominated at 15 satang, was printed by photogravure on zigzag-lined watermarked paper and perforated with a gauge of 13½.