About Bikram Sambat
Bikram Sambat (विक्रम सम्बत्) is Nepal's official calendar. It runs roughly 56 years and 8 months ahead of the Gregorian calendar, with each year beginning on 1 Baisakh, which falls on April 13 or 14.
BS uses solar months — the start of each month is fixed by the sun's entry into a zodiac sign (sankranti) — but Hindu festivals follow lunar tithis. That's why festivals like Dashain and Tihar shift between Gregorian dates from year to year.
The current authoritative source for the BS calendar is the Nepal Panchanga Nirnayak Samiti (Calendar Determination Committee), which publishes the official patro each February or March.