The Roman calendar was made up of twelve months and 365 days, but the movement of the Earth around the Sun lasted a little longer (365.25 days). This gap had produced a delay of 90 days, so the seasons -very important for the harvests- danced. Julius Caesar brought the wise Sosigenes of his retreat to Alexandria in 44 B.C. to undo the mess. Fixed by adding a day (bis sextus , leap year) every four years and one day was removed from February. In honor of Julius Caesar, the first month of summer was renamed Quintilis , by Julius . When Augustus proclaimed himself emperor, not to be outdone, he changed the name of the month Sextilis by Augustus and another day was removed from February, and with no one to stand up for him, he stayed at 28. The new calendar was renamed Julian .
This calendar worked correctly until in 1582 it was discovered that Sosigenes they were also a bit out of date; The translation period was not 365.25 days but 365.2422 days. It was little, but after centuries the gaps would have been noticed. So, Pope Gregory XIII organized a commission of wise men. The measures adopted by this commission, and approved in the bull Inter Gravissimas , there were two:
1.- All years that are multiples of 4 will be leap years, except those ending in 00 whose first digits are not multiples of 4. 1600 and 2000 were, but 1700, 1800, 1900 were not. A curious case is 4000 which, being a multiple of 4, the commission determined that it was not a leap year (¿?). This last fact, personally, I do not care much.
2.- Skip ten days in the calendar. It went from Thursday, October 4 to Friday, October 15. In the course of history these 10 days were lost.
This calendar, called Gregorian , is in force today and was gradually adopted by all countries; first the Catholic countries, in 1700 the Lutherans (with 11 days delay with respect to the new calendar), in 1752 the English... and the last, the Greeks in 1927 (with 13 days delay). This disparity of criteria led to Cervantes and Shakespeare dying on the same day -April 23, 1616- but 10 days apart... the English were governed by the Julian calendar and the Spanish by the Gregorian. So, when Shakespeare died in Spain, the calendar marked May 3 . There were also other anomalous circumstances, such as the fact that those who died on October 4 were not buried until October 15 -the ten days that were lost in history- or that a month of February had 30 days in Sweden .
Cervantes and Shakespeare
Sweden's ingenious idea to fix the error, and not have to suppress the ten days all at once, was to adapt gradually:starting from 1700, and suppressing the extra day of the next 11 leap years, they calculated that in 1740 they would already be equal to the new calendar. When they began to apply this brilliant measure, in 1700, they realized that at that time they were one day out of step with those who continued with the Julian and ten with those who were already applying the Gregorian, with the consequent problems in any commercial agreement or international politician. So, in 1712, they decided to straighten out the mess and return to the Julian... the day that they had removed in 1700 -in 1704 and 1708 they were no longer removed-, they put it back in 1712, also a leap year, so the month of February of 1712 in Sweden had 30 days. Thus they returned to the Julian, until in 1753 they adapted to the Gregorian, removing 11 days.