Asalha Bucha commemorates the first sermon delivered by Buddha, in which he explained his doctrine of the "Four Noble Truths" and is seen in the Buddhist faith as the day when "the wheel of dhamma was set in motion". Dhamma is the principle or law that keeps balance/order in the universe. The law was theorised and formulated by Buddha and over time the word "dhamma" has also come to mean his teachings, so essentially Asalha Bucha is celebrating the beginning of Buddhist teachings and of the religion itself.
The day after Asalha Bucha is the start of Buddhist Lent, known as "Khao phansa" or "Rains Retreat". It is when monks remain in their temple for a period of 3 months (during the rainy season) to pray, meditate and study Buddhist scripture.
Laypeople often participate in Khao phansa by following a vegetarian diet and giving up alcohol, tobacco and other luxuries. They also bring offerings to their local temple. The offerings traditionally include candles, food, bottled water and other basic necessities. The tradition of giving candles started in the days before temples had access to electricity. Monks would use the candles as a source of light so they could pray and study during the hours of darkness.
The town of Ubon Ratchathani, in Eastern Thailand, is famous for it's candle festival which is held each year during Asalha Bucha and Khao phansa.
Here are a few pictures from the candle festival.................