May 27 Vidakha Puja (observed)
Visakha Puja celebrates the Buddha’s birthday, enlightenment day, and passing away day. They all occurred in his life on the full-moon day in May, in different years of course. Therefore, this is one of the most important days in the Buddhist tradition, if not the most important. Visakha refers to the Visakha asterism, a Vedic construct, having to do with the day when the moon and the Visakha constellation align.
Ajahn Lee spoke about the way Thais honor this day: https://www.accesstoinsight.org/lib/thai/lee/visakha.html