Tomorrow is Lhabab Duchen according to most Tibetan calendars.
http://bhutanjournals.com/history-cultu ... a-heavens/
It is on this day that Buddha Shakyamuni descended to The Heaven of Thirty-Three Trayastrimsa in order to give teachings to benefit the gods in the desire realms, and to repay the kindness of his mother by liberating her from Samsara. This is considered to be one of the great deeds of the Buddha among eight great deeds. It is part of the Buddhist tradition to engage in virtuous activities and prayer on this day.
When Shakyamuni’s mother dies, she is reborn as a deva in the Trayastrimsha heaven which is presided over by the Brahmanical god, Indra. After the Buddha attains enlightenment, he goes to Trayastrimsha to teach the Abhidharma to his mother and other celestial beings. After three months of teaching in Trayastrimsha, the Buddha decides to return to his disciples and lay followers. His descent from the heaven takes place at Sankashya in modern Uttar Pradesh, India.