On February 29, Leap Day corrects the mismatch between our calendar year and Earth's orbit. Earth's orbit takes approximately 365.2422 days, not the neat 365 days our calendar counts, necessitating an extra day every four years to keep seasons consistent.
Originating from ancient efforts, the modern leap year system was refined by Pope Gregory XIII in 1582, improving upon Julius Caesar's initial leap year calendar. Without leap days, seasonal and calendar alignments would drift over centuries.
X / Dr. James O'Donoghue
Leap years don't occur strictly every four years; to maintain precision, years divisible by 100 are leap years only if also divisible by 400. This adjustment keeps our calendar in sync over long periods.
Despite its quirks, the leap year system remains crucial for aligning our calendar with Earth's orbital period, ensuring seasonal consistency, and marking years filled with significant events like elections and Olympics.