How do leap days keep a solar calendar aligned with the seasons?

A solar calendar tracks the tropical year—the time it takes for Earth to complete one orbit around the Sun—which is approximately 365.242 days long. Because a standard calendar year consists of 365 days, it is roughly a quarter of a day shorter than the actual solar year.123

Without adjustment, this discrepancy would cause the calendar to drift out of alignment with the seasons over time. Leap days function as a form of intercalation, where an extra day is added to the calendar to account for this accumulated fractional time.431

In the Gregorian calendar, this is managed by:
* Adding a leap day every four years.12
* Omitting the leap day in century years unless they are exactly divisible by 400 (e.g., 1600 and 2000 were leap years, while 1700, 1800, and 1900 were not).254

This precise rule ensures the calendar remains synchronized with the vernal equinox and the cycle of the seasons.153