How often is one calendar year identical with another?

Answer 1

It is either every 6th, 11th or 28th year, depending on how adding an extra day in a leap year affects it. Without leap years, a calendar would always repeat every 7th year. If in a 7 year period there is one leap year then one day will be skipped, so the next repeat comes around in 6 years. So if you take from 2013 to 2019 inclusive, 2016 is the only leap year of those 7 years. 2013 and 2019 have identical calendars. The pattern is that a leap year repeats its calendar every 28 years. The year after a leap year repeats in 6 years. Two years after a leap year, which is also two years before a leap year, repeats every 11 years and 3 years after a leap year, which is also one year before a leap year, repeats every 11 years. 2012, a leap year, will repeat in 2040. 2013, a year after a leap year, repeats in 6 years, in 2019. 2014, two years after/before a leap year repeats 11 years later in 2025 and 2015, a year before a leap year will repeat 11 years later in 2026. 2013 and 2019 are repeats, but 2014 and 2020 aren't, because unlike 2014, 2020 is a leap year. If a leap year causes a day to be skipped, then it will come around again in the next cycle, so then you get an 11 year gap. A leap year itself is repeated every 28 years.