Addendum: The #Amazigh (AKA #Berber) people use a variant of the #JulianCalendar to date traditional festivals; in this calendar, #LeapDay is added to the last month rather than the second one. Amazigh in #Morocco and #Libya celebrate traditional New Year the day after those in #Algeria, where it is a national holiday.

In 1900, Maksim Trpković revised the #JulianCalendar. Under his system, an year that yields a remainder of 0 or 400 when divided by 900 is a #LeapYear.

Milutin Milanković slightly altered this in 1923, changing the rule to years that give a remainder of 200 or 600. A mean year in this calendar is just 2 seconds off the #TropicalYear. It is used by some, but not all, #EasternOrthodox churches.

The #EasternOrthodox Churches never adopted the Gregorian calendar (except in #Finland). While Orthodox-majority countries use the Gregorian calendar for civil purposes, many still use the #JulianCalendar for religious purposes. You have probably heard about #UkrainianRefugees celebrating #Easter and #Christmas on different dates to those of #Catholics and #Protestants; this is because the Ukrainian Orthodox Church still uses the Julian Calendar (for now).

In the 17th century, #IsaacNewton recognised that the #julianCalendar was deficient but the #GregorianCalendar was too #Catholic for him. He worked on a new calendar whose mean year was just a few seconds shorter than the #TropicalYear (the mean Gregorian year is about 29 seconds longer). Newton viewed this as a feature rather than a bug, because under his system, the dates of the solstices and equinoxes would gradually drift back to those when #Jesus was alive.

A variant of the #julianCalendar was used in #Ottoman #Türkiye, in which the year began on 1 March. The newly-formed Republic of Türkiye officially adopted the Gregorian calendar in 1926 as part of #Atatürk's policy of modernisation and westernisation.

While #Britain was still using the #julianCalendar, #Catholics in #Ireland had to seek permission from the #pope to celebrate festivals on the wrong days because Catholicism was, to all intents and purposes, illegal at the time.

#PenalLaws

The #GregorianCalendar is the one used in most of the world today. It was designed by #ChristopherClavius and #AloysiusLilius, but like the #JulianCalendar, it was named after the bloke who commissioned it.

The main difference is that in the Gregorian calendar, years that are integer multiples of 400 are not leap years.

But, as we now know, the #JulianCalendar's mean year is about 11 minutes 15 seconds longer than the #TropicalYear, and by the 4th century AD, the dates of the #solstices and #equinoxes had drifted from their official dates by several days.

In the 16th century, #Pope Gregory XIII used his authority as #PontifexMaximus to reform the calendar again, because that is something #popes can do.

The new calendar was named the #JulianCalendar after its commissioner, #JuliusCaesar, even though #Sosigenes is the one who invented it. It was one of the better algorithmic calendars at the time, and spread throughout the ancient world.

Today is the end of the tax year in the #UK. If you’ve ever wondered why it’s 5 April, which seems rather arbitrary, it’s related to being historically new year’s day in the ecclesiastical calendar or 25 March in the #JulianCalendar. In the #GregorianCalendar, it becomes 5 April.