There are different kinds of calendars:
A solar calendar is based on the earth's movement around the sun. It takes 365.2422 mean (or "average" )solar days for the earth to make one complete orbit round the sun. A common year has 365 days. Thus, there is a difference of approximately 6 hours every year. As a first approximation, in Julian calendar (named after Julius Caesar), every fourth year is declared as a "leap" year with 366 days. The "extra" day in added to February. When we were children, we had to recite:
Calendars evolve over time. In the old Roman calendars, September, October, November and December used to be the seventh, eighth, ninth and tenth months as indicated by the corresponding prefix. Folklore says that August is named after Augustus (aka Octavius) Caesar, but the month originally had only 30 days; so one day was "borrowed" from February to make it 31 to match the number of days in July, which is named after Julius Caesar. That is probably why a day is given back to February.
A lunar calendar is based on the moon's movement round the earth. It takes approximately 29.5 days for the moon to make one complete orbit. A lunar month usually has 29 or 30 days.
The Burmese calendar is a luni-solar-socio-religious calendar. A Burmese year is generally made up of 12 months with alternating months having 29 and 30 days. This means that the "pure" lunar calendar has 355 days. Every three years, an intercalary month called the Second Waso is added to keep the lunar calendar back in sync with the solar calendar. In such years, the Buddhist lent (or vassa) begins on the Full Moon day of the Second Waso.
Occasionally, an extra day is also added to the month of Na-yone. Such a year, with the one month and one day longer than a normal year, is called "war gyi thut" year.
The Burmese New Year, which is based on the Fixed Zodiac system, falls on or around April 16. Thingyan means change. Cula thingyan (or small change) occurs every month. Maha thingyan (or big change) occurs once a year. The Burmese consider it to occur in Meiktha on or around April 13 and lasting three or four days. Visitors to Myanmar fondly remember the Thingyan water festival.
Each Burmese month has an associated festival. See the verses on the TBSA invitations for the Burmese festivals such as Hta-ma-ne pwe.
85% or more of the people in Myanmar are Buddhists. Important events in the life of Buddha fall on the Full Moon days. TBSA celebrates minimally, in Theravada tradition, the Full Moon days of Kason, Waso, and Thadinkyut.
The Persians celebrate their New Year on or around March 21, which is
also known as the Vernal Equinox (or the First Day of Spring) according
to the Moving Zodiac system. In a sense, the difference between the Moving
Zodiac and Fixed Zodiac systems is due to a factor known as the "precession
of the equinoxes."