Following the ancient Indian practice, Buddhism adopted the lunar calendar (dinadassana). According to this calendar the new year starts on the full moon of March-April and the names of the months are Citta, Vesakha, Jeññha, âsaëha, Savana, Poññhapàda, Assayuya, Kattika, Maggasira, Phussa, Magha and Phagguna. There are two significant days each month called uposatha, the 1st and 15th days, during which devout Buddhists will renew their commitment to the Dhamma, make a stronger effort than usual to practice it and perhaps also keep the eight Precepts. On these days too monks meet together to recite the Patimokkha. All Buddhist countries used the lunar calendar until the 19th century, when it was replaced by the western Gregorian calendar. In the 1950’s Sri Lanka returned to the lunar calendar but this caused considerable confusion and the Western system was soon reintroduced. The date of the Buddha’s birth has never been used by all Buddhists as the beginning of an era because there has never been complete agreement about when he was born.
- http://www.BuddhismA2Z.com/ Buddhism A to Z. Ven. Dhammika, 2007.