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The dates of Chinese festivals are calculated according to the Chinese lunar calendar, which comprises 12 lunar months of either 29 or 30 days. Each lunar year therefore lasts no longer than 355 days, ten days less than the Gregorian year. To keep the two calendars in synchrony with one another, it became necessary to ‘add’ an extra month every two or three years, in a manner similar to the convention of creating leap years. Known as ‘intercellary months’, these additional months ensure that the Lunar New Year continues to be celebrated at approximately the same time year after year.

Because the lunar year ‘ends’ before the Gregorian year, the subsequent lunar year ‘starts’ before the old Gregorian has come to an end. This has the effect of causing the New Year to fall on progressively earlier days. If this continued indefinitely, unfortunate Chinese would find themselves celebrating the New Year earlier and earlier. If the difference between the two calendars was allowed to grow, there would be years when the Lunar New Year would fall during the December of the previous year, or even earlier.

However, every three years, the difference between the two calendars is compensated for by the addition of a intercellary month. After every ‘leap’ year, Lunar New Year would once again occur later in February, before falling on earlier dates in subsequent years.

Calculations for the dates of the New Year from 1864 to 2043 have been made and are available in the book The Comparative Solar and Lunar Calender. Within this time period, the earliest CNY recorded falls on 22 January (1879, 1898, 1909, 2004) while the latest is on 20 February (1920, 1985)

Reference:

All Rights Reserved. Singapore: Landmark, 1997

Title: The comparative solar and lunar calendar (864-2043)
Author: Goh, Kee Seah 
Call No.: RSING English 529.3 GOH