why this was happened
Answers
In 46 BC, Julius Caesar noticed that Egyptians used a calendar based on the sun. He created his own system, the Roman or Julian calendar, to solve the problem of the drifting seasons. Each solar year in Caesar's calendar had 365 days and an additional six hours. He adjusted this by adding an extra day (leap day) every four years, to the shortest month.
However there was one small problem. Caesar's estimation of a year was off by 11 minutes every year. By 1582, this had accumulated to a 10 day discrepancy. It would affect Christian festivals such as Easter that was set by the moon. Pope Gregory XIII decided to fix the issue. He issued a Papal bulletin by which Oct 4 was followed directly by Oct 15 and the time in between was erased on the calendar and declared non-existent by the Pope.