The next leap day is Tuesday, 29 February 2028, 535 days from today. 2028 is a leap year because it divides by 4 and is not a century year, giving February 29 days and the year 366.
A leap day is added because Earth takes about 365.2422 days to orbit the Sun, not a round 365. Adding one day every four years overcorrects slightly, so century years are skipped unless they divide by 400 — which is why 1900 was not a leap year but 2000 was.