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, Vesākha,Jeṭṭha, Āsāḷha, Sāvaṇa, Poṭṭhapāda, Assayuja, Kattika, Māgasira, Phussa, Magha and Phagguna. Starting with Sunday, the days of the week are Ravivāra, Candavāra, Kujavāra, Budhavāra, Guruvāra, Sukkavāra and Sanivāra.
There are three seasons of four months each; summer (gimhāna), the rainy season or monsoon (vassāna) and winter (hemanta). Each month is further divided into two fortnights (aḍḍhamāsa), one illuminated by the moon (sukkapakkha) and the dark half (kālapakkha). On full moon and half moon days monks meet together to recite the Pātimokkha. The 1st, 8th, 15th and 23rd days of each month were called uposatha and on these days devout Buddhists would renew their commitment to the Dhamma, make a stronger effort than usual to practise it and perhaps also keep the eight Precepts. This practice is still followed today.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.
An important feature of any religious calendar is the holidays (nikkammadisāva), i.e. holy days, when significant events in the history of the religion are celebrated. People tend to be more pious and careful in the practice of their religion on such days. While this is admirable, the sincere Buddhist will try to be virtuous, kind, charitable and mindful all the time. The Buddha said: ‘The pure consider every day special, for them every day is holy.’ (M.I, 39). See Time.