Today, October 4th, 1582 was the last day of the Julian calendar. To sync to the Gregorian calendar, 10 days were skipped and the next date was Oct 15.
The Julian calendar was proposed by Julius Caesar in 46 BC and was put into use on January 1, 45 BC. It was very close to the actual length of the year but it was found to drift and left almost a day in a period of 128 years because it was 365.25 days long. By 1582 it had drifted a full 10 days from the actual date.
Pope Gregory XIII introduced the Gregorian calendar in 1582, because the Julian calendar was becoming inaccurate, especially in the calculation of the date of Easter. The move to reform it had started in the reign of Pope Paul III in 1545. The German mathematician Christopher Clavius was the architect of the Gregorian calendar.
Ten days were omitted from the Julian calendar, and the day following October 4, the day on which Gregorian calendar was adopted, was October 15 in 1582. It made the average year 365.2425 days long, approximating the 365.2422-day tropical year based on the Earth’s revolution around the Sun. The rule for leap years is:
“Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.”
The Gregorian calendar was gradually adopted by catholic countries around the world. Protestant and Eastern Orthodox countries also adopted it in the next few centuries with Greece being the last Orthodox country to do so in 1923. Great Britain and its colonies adopted it in 1752. Now the calendar is used all over the world and also carries secular names such as “Common Era”.