Constantine I born February 27, 272 – May 22, 337
The **Julian calendar **is a calendar that was invented by Julius Caesar, the dictator of Ancient Rome, in 46 BC.
The calendar has a regular year of 365 days divided into 12 months, and a leap day is added every four years, making the average Julian year 365.25 days. The calendar was still used by some countries even in the 20th century, and is still used by many national Orthodox churches. The problem is that this calendar creates too many leap days, which means that it gains a day about every 128 years.
In the 16th century, the **Gregorian calendar **was introduced in Europe because it was more accurate with regard to the length of the year, and it also moved the date for the vernal equinox, the first day of spring (or of fall in the Southern Hemisphere) to where it belonged again, on March 21. The Gregorian calendar has 97 leap days every 400 years, while the Julian had exactly 100.
People sometimes use the term Old Style or O.S. to refer to the Julian calendar, with N.S. or New Style referring to the Gregorian calendar
The calendar was made official by Pope Gregory XIII, for whom it was named, on February 24, 1582.