Why was the year 1500 a leap year?

Answer 1

Because 1500 is a century , so we should check whether it is divisible by 400 not by 4 alone. Since it is not divisible by 400 its not a leap year.

1500 WAS actually a leap year, the Gregorian calendar didn't commence until the year 1600.

Answer 2

because the Gregorian calendar was not in use until 1582

According to the Julian calendar, the predecessor to the Gregorian calendar, 1500 was a leap year, but according to the Gregorian calendar it is not.