Gregorian Calendar Day (Guregorio-reki Seitei Kinenbi)

What Is Gregorian Calendar Day?

October 15 is "Gregorian Calendar Day" in Japan. In 1582, Pope Gregory XIII established the Gregorian calendar to reform the Julian calendar then in use, and it took effect on October 15 of that year. It marks the beginning of the calendar used today in Japan and most countries around the world.

The "Lost Ten Days" and the Leap Year Rule

Switching to the Gregorian calendar was a major reform to realign the calendar with the seasons.

• October 5–14, 1582 never existed: The Julian year was slightly longer than the actual year, so over centuries the calendar had drifted about ten days from the seasons. To fix this, the day after Thursday, October 4, 1582, became Friday, October 15, skipping ten days while keeping the sequence of weekdays intact.
• 97 leap years every 400 years: The Julian calendar had a leap year every four years without exception, but the Gregorian calendar skips years divisible by 100 but not by 400 (such as 1700, 1800, and 1900). Reducing leap years to 97 per 400 years shrank the error to about one day in a little over 3,000 years.

Countries Switched at Different Times

• Britain in 1752: Catholic countries switched quickly, but Britain and its colonies adopted the calendar only in 1752, when September 2 was followed by September 14.
• Russia in 1918: Russia adopted it after the revolution, in 1918. That is why the "October Revolution" took place in November by the Gregorian calendar.

Calendar Trivia

• Japan's switch: An imperial edict issued on the 9th day of the 11th month of Meiji 5 (1872) declared that the day after the 2nd day of the 12th month would be January 1, Meiji 6 (1873). Less than a month's notice meant December nearly vanished, and there is a well-known story that government officials were not paid for that month.
• Easter: One purpose of the reform was to calculate the date of Easter, an important Christian feast, correctly.
• The length of a year: The tropical year (one cycle of the seasons) is about 365.2422 days. The average Gregorian year is 365.2425 days — very close.
• 2000 was a leap year: It is divisible by 100 but also by 400, so it was a leap year. The next century year that is not a leap year will be 2100.

Ideas for Gregorian Calendar Day

• Open your planner and review your upcoming schedule.
• Use "October 5–14, 1582 never happened" as a conversation starter.
• Look up events and seasonal words from Japan's old lunisolar calendar to understand the origins of traditional observances.
• Use a perpetual calendar to find which day of the week you were born.

Fix Small Gaps Early

October 15 is the day a drift that had built up over centuries was fixed by boldly skipping ten days. Thanks to this reform, we can still plan with the same calendar today. Small gaps grow when left alone — the same may be true of work schedules. Today, take a moment to review your calendar.