Switch your calendar to October 1582 and take a look – you’ll see something resembling the above image. There is a simple explanation for this though. In the year of 1582 the Pope decided that to correct the accumulation of time over the year they would remove the period of time from 5th October to 14th October to compensate.
Pope Gregory XIII, whom the modern calendar is named after (Gregorian), also established a system to prevent the accumulation from developing all over again. Every four years divisible by four would continue to be a leap year, but years divisible by 100 would not, unless they are also divisible by 400. The year 2000 was one of these special years that comes once every four centuries. 2000 is divisible by 100 but also divisible by 400 and therefore it is designated as a leap year.