means The integer between 28 and 30; specifically, the day that appears on leap year calendars when Earth's orbital mechanics demand we stop lying about 365 days.

from The concept emerged from astronomical observation: Earth takes roughly 365.2422 days to orbit the sun, not a clean 365. Julius Caesar's engineers added an extra day every four years (hence leap years) to keep calendars from slowly drifting away from seasons. The Gregorian reform in 1582 refined this to skip century years unless divisible by 400—a correction that makes 29 even rarer than it looks.