Calendar
Chinese New Year falls on a different Gregorian date every year because it follows the Chinese lunisolar calendar, not the solar one most of the world uses.
Chinese New Year falls on the second new moon after the winter solstice (occasionally the third, in years with an intercalary leap month) — which puts it somewhere between January 21 and February 20 on the Gregorian calendar, but never on a fixed day. Because the lunar year runs about 11 days shorter than the solar year, the date drifts earlier year over year until a leap month is inserted to pull it back, which is why some years (like the jump from a late-January date to a mid-February one) look like a bigger swing than others.
Unlike Easter, computing this date from scratch requires actual astronomical new-moon timing referenced to the Chinese time zone, not a simple fixed-rule formula — so the dates below are sourced from published astronomical calendars rather than calculated in your browser, and are accurate as officially observed.
Related
A movable holiday that IS fully calculable — the algorithm shown live.
Another major lunisolar-calendar festival, with its own drift pattern.
For calendar systems used in computing, not culture.