The major holidays and special days of observance in Theravada Buddhism are listed below. Typically on these days, lay followers will go to the temple to make offerings of gifts, food, or other necessities to the monastic monks or nuns. They will listen to Dhamma talks, renew precepts, take the 8 precepts, and practice meditation too.
Makha Bucha (also Magha Puja) also known as "Sangha Day" celebrates when 1,250 Arahants met nine months after the Buddha's enlightenment and where the Buddha gave important Dhamma instructions. It usually occurs in February.
Buddha's Birthday is known as Vesakha and is the most important festival of the year. It is celebrated on the first full moon day in May, but when it's a leap year it falls in June. It celebrates the birth, passing (paranibbana), and enlightenment of the Buddha.
Asalha Puja Day
Also known as "Dhamma Day" celebrates the Buddha's first teaching on the full moon day of the 8th lunar month, approximately July.
This robe offering ceremony is held on any date within the end of the Vassa Retreat. New robes and other requisites can be offered by the laity to the monks. It is usually in October or November.
Celebrated on the Full Moon day in December, this important day commemorates when the Arahant Ven. Sanghamitta (daughter of King Ashoka) went to Sri Lanka to teach Buddhism and set-up the bhikkhuni Order in Sri Lanka. She also went there with a sapling of the Bodhi Tree. This tree survives to this day, see: Jaya Sri Maha Bodhi.
Holidays in Modern usage
In modern times in many non-Asian countries, it is not always feasible to celebrate all of the above due to work and family time constraints and the lack of recognition for these holidays in non-Buddhist countries. Some Buddhist communities have made only the Full Moon day as the Uposatha day each month and then celebrate the rest of the above at the temples on the nearest Sunday to the Full Moon or actual holiday date.
To limit the number of celebrations further in perhaps a Modern Theravada way, Vesakha should be celebrated since it is the most important Theravada holiday, the Kathina if there are monks or nuns in the community, and then only one other one for the year. In the spirit of recognizing the great contributions of female disciples to Buddhism, Sanghamitta Day could be the second or third holiday to be observed and it could be done in conjunction with Christmas in non-Buddhist countries. Sanghamitta Day is especially relevant, not only to honor female practitioners, but because it represents one of the first major moments of Buddhist missionary work outside of India, an appropriate holiday for Buddhists in non-Buddhist lands.