Losar falls on the first day of the first month of the Tibetan calendar, and that calendar behaves in a way no other in common use does.
It descends from Indian astronomy transmitted through the Kalachakra literature and was reworked in the fifteenth century by the Phugpa tradition, whose computations are still the standard. It is luni-solar, so months follow the moon and an intercalary month keeps the year with the sun. That much is familiar.
What is not familiar is how a date is assigned. The unit is the tithi, a thirtieth of a lunation, which is a fraction of the moon's motion rather than a period of daylight and does not line up with days. The Tibetan rule gives a calendar day the number of whichever tithi is running at daybreak. Because a tithi is slightly shorter or longer than a day depending on the moon's speed, two consequences follow:
- Sometimes a tithi begins and ends entirely between two daybreaks. No day gets that number, and the date is skipped, a month can go from the ninth to the eleventh.
- Sometimes a tithi spans two daybreaks. Two consecutive days get the same number, and the date is doubled.
So a Tibetan month has a variable number of days and calendars are printed with the omissions and duplications shown.
Where this puts Losar
Tibetan and Chinese new year are computed by different systems from a shared astronomical ancestry, so they land near one another without being tied. Usually they coincide or fall a day apart; in some years the intercalation falls differently and Losar comes a full month after Chinese New Year.
There is more than one Losar in any case. Tamu Losar and Sonam Losar, kept by the Gurung and Tamang communities in Nepal, fall on separate dates entirely; Sikkim keeps Losoong at the end of the tenth month; Bhutan's observances vary by district. New year in the Himalaya is a set of festivals, not one.
The festival itself is generally held to predate Buddhism in Tibet, with an agricultural and Bon layer, the year-end purification, the throwing away of the old year's residue by torchlight, under the monastic ceremony.