Full Moon days are important for Buddhists as major events took place on these days in the time of the Buddha.
The Buddha-to-be was conceived in the womb of Maya, chief queen of King Suddhodhana, the ruler of Kapilavattha, on the Full Moon Day of Savana (July/August), the fifth month of the Buddhist Calendar, in the Great Era 67. He was born on the Full Moon Day of Vesakha (April/May), the second month in the Buddhist calendar, in the Great Era 68, so the Vesak Celebration of Buddhists is celebrated on the Full Moon Day, also called the Buddha’s Birthday Celebration. However, it is just called Vesak Day, perhaps to respect the Buddha since the celebration of birthdays is a secular affair. Vesak is named after the month (Vesakha) when the Buddha was born. On the same day (Full Moon Day of Vesaka) in the Great Era 103 the Buddha attained enlightenment. He also passed away on the Full Moon Day of Vesakha in the Great Era 148.
Moreover, two months after his enlightenment, on the Full Moon Day of the month Asalha, the fourth month of the Buddhist Calendar (June or July), the Buddha preached his first sermon (Dhammacakkapavattana sutta). On the Full Moon Day of the month Assayuja, the seventh month of the Buddhist Calendar (September or October), at the end of his first vassa, the Buddha’s missionary ministry was initiated with 60 disciples. On the Full Moon Day of Magha, the eleventh month of the Buddhist calendar, in the Great Era 103, Ven Sariputta and Ven Moggallana were appointed as the Buddha’s chief disciples, his left hand disciple and right-hand disciple respectively; and on this Full Moon day, fifteen days after his ordination, Ven Sariputta attained arahantship while Ven Moggallana's attainment of arahantship was achieved seven days earlier. In the Great Era 148 on the Full Moon Day of Kattika (October/November), the eighth month of the Buddhist calendar, Ven Sariputta passed away and two weeks later, on the New Moon Day of Kattika, Ven Moggallana passed away.
Since the time of the Buddha, Patimokkha (monastic rules for monks) recitation has been made by the Buddhist Sangha, fortnightly, on Full Moon Days and New Moons days. This practice is called uposatha. Buddhist devotees took the eight precepts (uposathasila) on these days. Full Moon Days are public holidays in Buddhist countries.
Now, at Mangala Vihara (MV), we celebrate Full Moon and New Moon Days with important activities. These activities are wholesome deeds or meritorious acts. When flowers, candles, etc. are offered to the Buddha, special chanting is performed, special lunch donations are served to members and devotees, and special blessings and evening dhamma talks are delivered by resident monks.
Actually these good deeds can be done anywhere at any time. So, why are these done on Full Moon and New Moon Days only? The answer is simply that we cannot do these good deeds every day. So Full Moon and New Moon Days are selected for the performance of these meritorious deeds.
Another reason or explanation is as follows: According to the Catumaharaja-sutta of Anguttara Nikaya (Tikanipata, The Threes), ministers of the four great kings of gods namely Ratarattha, Virulaka, Virupakkha, and Kuvera, come down to the human world on the eighth waxing and waning days; sons of the kings of gods come down to the human world on the fourteenth new moon days; and the kings of gods themselves come down to the human world on the fifteenth Full Moon and New Moon Days.
In this regard it should then be noted that in the Buddhist calendar there are the two types of New Moon Days: one falls on fourteenth waning day which is the end of month, and the other falls on the fifteenth waning day which is also the end of month. Full Moon Days fall in the middle of a month and always upon the 15th day of the waxing of the moon. There are 12 calendar months and the months have alternate 29 days and 30 days. The first month of the calendar has 29 days, the second month has 30 days, and so on. And days are counted from 1 to 14 or 15, not from 1 to 29 nor 1 to 30 as in other calendars. For example, days of the calendar are waxing 1 to 15 and waning 1 to 14 or 15. Thus in the Buddhist calendar, the New Moon Day (and the end of the month) may fall on either the 14th or 15th waning day of the moon.
These celestial beings observe the human world. They want to know if human beings are performing meritorious deeds and thus may be reborn in the deva world and populate it with merit makers. That is why at Mangala Vihara Full Moon and New Moon Days are selected for the performance of special meritorious deeds so that our wholesome deeds can be recognized by celestial beings who, as it is traditionally believed, regularly come to the human world to observe us.
One of distinguished MV’s celebrations is Pin Kamma, celebrated on Full Moon and New Moon Days. Pin Kamma is a Sri Lankan word derived from Pali. The original Pali word for Pin Kamma is Punna Kamma that means wholesome (Punna) act (Kamma) or meritorious (Punna) deed (Kamma). In Sri Lanka almost all Buddhist religious ceremonies are called Pin Kamma or Pin Kama (Punna Kamma). For example, there are ceremonies of Vesak Pin Kamma (Vesak Day or Buddha Day Celebration), Kathina Pin Kamma (Kathina Robe Donation Ceremony), Seventh Day Pin Kamma (Transferring Merit to a Departed One on the Seventh Day Counted from the Day on which he or she passed away), and Bana Pin Kamma (Ceremony of Buddhist Religious Sermon). Actually all the Buddhist ceremonies might be named Pin Kamma. Very often Kamma is spelled with only one ‘m’; that is, Pin Kama.
At Mangala Vihara this name (Pin Kamma) is only applied to the meritorious deeds that are done on Full Moon and New Moon Days. More specifically, the lunch donations to the resident monks, members and devotees made on those days are called Pin Kamma. This is one of the Theravada practices of Mangala Vihara, initiated by the late founder Bhante, MM Mahavira who was a Sri Lankan monk. It is believed that the founder gave the Sri Lankan name of the meritorious deed (Pin Kamma) to the lunch donations at Mangala Vihara. This name is given to this merit only and thus this has become significant.
In conclusion, Full Moon Days and New Moon Days are very important, as has been described. Extraordinary meritorious deeds which cannot easily be performed every day are reserved for these days.