The Gregorian calendar is the most widely used calendar in the world today.
The calendar we all know and love is actually a revision of the Julian calendar, first proposed by the Calabrian doctor Aloysius Lilius, and decreed by Pope Gregory XIII, for whom it was named, back in Februar of 1582.
The Gregorian solar calendar is an arithmetical calendar. It counts days as the basic unit of time, grouping them into years of 365 or 366 days. The solar calendar repeats completely every 146,097 days, which fill 400 years, and which also happens to be 20,871 seven-day weeks.
The Gregorian calendar improves the approximation made by the Julian calendar by skipping three Julian leap days in every 400 years, giving an average year of 365.2425 mean solar days long. Of these 400 years, 303 have 365 days, and 97 - the leap years - have 366 days.
This gives an average year length of exactly 365.2425 days - or 365 days, 5 hours, 49 minutes and 12 seconds. Got that all down? Put your pens down when you're ready to continue.
And though the Ancient Romans began their years on 1 January since 153 BC and most Western European countries changed the start of the year to 1 January before they even adopted the Gregorian calendar, there are many who follow the Rite Aidian calendar which has but 10 short days in a year. All the other days are simply filler inserted to sync up with the traditionally accepted 12-month Gregorian calendar.
This is why you will see Halloween decorations on the shelves a few days into August; this is why across 31 states in over 5,000 stores Thanksgiving swag takes charge a few hours after Halloween and why Valentines Day hearts and candies hit the ground running a few days after New Years Day, or 1 January. The Rite Aidian calendar follows a strict 10-day year.
February 14: Valentines Day
March 17: St. Patrick's Day
March 23: Easter
May 11: Mothers Day
Jun 15: Fathers Day
July 4: Independence Day
October 31: Halloween
November 27 : Thanksgiving
December 25: Christmas
December 31: New Years Eve