Intercalary days are extra days added to a calendar to keep it in sync with the solar year. Most calendars, like the Gregorian calendar, have a fixed number of days, but the actual time it takes for the Earth to orbit the Sun is about 365.24 days. To account for this discrepancy, intercalary days are inserted periodically. A well-known example of intercalary days is February 29, which occurs every four years during a leap year. This adjustment helps ensure that seasonal events, such as the start of spring, remain consistent over time.