The Gregorian calendar Pope Gregory XIII

detail of tomb of pope gregory xiii celebrating introduction of gregorian calendar.

pope gregory xiii best known commissioning of calendar after being authored calabrian doctor/astronomer aloysius lilius , aid of jesuit priest/astronomer christopher clavius made final modifications. reason reform average length of year in julian calendar long – treated each year 365 days, 6 hours in length, whereas calculations showed actual mean length of year less (365 days, 5 hours , 49 minutes) result, date of actual vernal equinox had (over course of 13 centuries) slipped 10 march, while computus (calculation) of date of easter still followed traditional date of 21 march.

this verified observations of clavius, , new calendar instituted when gregory decreed, papal bull inter gravissimas of 24 february 1582, day after thursday, 4 october 1582 not friday, 5 october, friday, 15 october 1582. new calendar duly replaced julian calendar, in use since 45 bc, , has since come universal use. because of gregory s involvement, reformed julian calendar came known gregorian calendar.

the switchover bitterly opposed of populace, feared attempt landlords cheat them out of week , half s rent. however, catholic countries of spain, portugal, poland, , italy complied. france, states of dutch republic , various catholic states in germany , switzerland (both countries religiously split) followed suit within year or two, , hungary followed in 1587.

however, more century passed before protestant europe accepted new calendar. denmark, remaining states of dutch republic, , protestant states of holy roman empire , switzerland adopted gregorian reform in 1700–01. time, calendar trailed seasons 11 days. great britain , american colonies reformed in 1752, wednesday, 2 september 1752 followed thursday, 14 september 1752; joined last protestant holdout, sweden, on 1 march 1753.

the gregorian calendar not accepted in eastern christendom several hundred years, , civil calendar. gregorian calendar instituted in russia bolsheviks in 1917, romania accepted in 1919 under king ferdinand of romania (1 november 1919 became 14 november 1919), turkey in 1923 under atatürk, , last orthodox country accept calendar greece in 1923.

while eastern orthodox national churches have accepted gregorian calendar dates feast days occur on same date every year, dates of movable feasts (such easter) still calculated in eastern orthodox churches reference julian calendar.