"The Buddhist holiday of Sangha Day is called Makha Bucha Day in Thailand. It is observed in Thailand on the full moon of the third lunar month, which is usually the end of February or early March. In most other parts of Asia, Buddhist observe Sangha Day about a month later.
Sangha Day commemorates a day when 1,250 monks, all from different places and on their on initiative, spontaneously came to pay homage to the historical Buddha.
In Pali, Makha is the name of the third lunar month, and Bucha means "to venerate." On this day, Thai Buddhists gather at sunset in their local temples to participate in candlelight processions called Wian Tian. Ceremonies express appreciation for the order of monks founded by the Buddha and for the Three Jewels -- the Buddha, the Dharma, the Sangha."
This text is from About.com - Buddhism.