why do we have leap years

You waited for a human reply when AI could've answered first.

We have leap years because Earth's orbit doesn't take a whole number of days. One trip around the Sun (the tropical year, measured from one spring equinox to the next) lasts about 365.2422 days, not 365.

If every calendar year had 365 days, the calendar would slip by about 6 hours a year. That's roughly a day every four years and close to a month per century, so the seasons would gradually move out of line with the months.

The fix:

• Julian calendar (46 BC): Julius Caesar added a leap day every 4 years, making the average year 365.25 days. That's about 11 minutes too long per year, and by the 1500s the calendar had drifted around 10 days.
• Gregorian calendar (1582): Pope Gregory XIII adjusted the rule. A year divisible by 4 is a leap year, except century years, which only count if they're divisible by 400. That's why 2000 was a leap year but 1900 wasn't, and 2100 won't be.

The Gregorian rule gives an average year of 365.2425 days. That's off by only about one day every 3,000 or so years.

The leap day goes on February 29 because February was already the short, adjustable month in the Roman calendar.

That's the whole answer, and no friends were bothered in the making.