How the Bikram Sambat calendar works

Bikram Sambat (विक्रम संवत्, also Vikram Samvat) is the official calendar of Nepal. It is a sidereal solar calendar: each month begins when the Sun enters the next sidereal zodiac sign (its sankranti), and the year begins with Baishakh in mid-April.

#	Month	नेपाली	Begins around
1	Baishakh	बैशाख	mid-April
2	Jestha	जेठ	mid-May
3	Asar	असार	mid-June
4	Shrawan	साउन	mid-July
5	Bhadra	भदौ	mid-August
6	Ashwin (Asoj)	असोज	mid-September
7	Kartik	कात्तिक	mid-October
8	Mangsir	मंसिर	mid-November
9	Poush	पुस	mid-December
10	Magh	माघ	mid-January
11	Falgun	फागुन	mid-February
12	Chaitra	चैत	mid-March

Because the Sun’s apparent speed varies through the year, months have 29 to 32 days: the summer months (Jestha to Shrawan) are long, and the winter months (Poush to Falgun) are short. The year has 365 or 366 days, but there is no leap-year rule; the length follows from the month lengths.

The BS year is 56 or 57 years ahead of the Gregorian year: BS 2083 runs from 14 April 2026 to 13 April 2027.

Who decides the month lengths

Each year’s calendar is decided and published in advance by the Nepal Panchanga Nirnayak Bikas Samiti (Nepal Calendar Determination Development Committee), a body of the Government of Nepal. The committee’s decision is what makes a date legally and socially correct, so a formula cannot replace it. See The calendar data and Why sambat is exact or nothing.

Day numbers and weekdays

A BS day is the same civil day as its Gregorian equivalent, so weekdays are the same and sambat.date.toordinal() returns the standard library’s proleptic Gregorian ordinal. Nepali calendars traditionally start the week on Sunday (आइतबार); sambat.calendar defaults to Monday like the standard library, and accepts firstweekday=SUNDAY.