Lunar New Year's Day

What Lunar New Year's Day marks

Lunar New Year marks the beginning of a new year according to lunar calendars or, informally but commonly, to lunisolar calendars. Because a year of twelve lunar months is about 11 days shorter than a solar year, lunar cycle-based calendars may have strategies to take this fact into account. Pure lunar calendars have twelve lunar months invariantly and consequently their New Year is not fixed relative to the solar year; no adjustments are made. In contrast, most lunisolar calendars also have twelve lunar months, but every few years, a thirteenth "leap month" is added to resynchronise with the solar year. Consequently, neither type of calendar begins on a fixed date in the international Gregorian calendar.

Background summarised from Wikipedia — Beginning of a year in a lunar calendar, available under CC BY-SA.