In 1582 Pope Gregory XIII introduced new Gregorian calendar that should replace old Julian calendar in the catholic countries across the Europe. The motivation for the adjustment was correcting the calendrical errors and the date of the vernal equinox. These alterations to the calendar were designed to prevent future drift and allow more consistent and accurate scheduling of the feast of Easter. Astronomers Aloysius Lilius and Christopher Clavius prepared this reform by the request of Pope Gregory XIII. Papal bull "Inter gravissimas" dated 24 February 1582 contained results of their hard work.

Gregory dropped 10 days to bring the calendar back into synchronization with the seasons. New leap year rule was introduced as well as original and practical scheme for adjusting the epacts of the moon when calculating the annual date of Easter. Four Catholic countries — Spain, Portugal, the Polish–Lithuanian Commonwealth, and most of Italy implemented the new calendar in 1582. Papal bull specified that Julian Thursday, 4 October 1582, was followed by Gregorian Friday, 15 October 1582. That's why dates from October 5th till October 14th 1582 never existed in those countries' calendars.

British people were not happy with those changes and even riots occurred. It is said that rioters demanded "Give us our eleven days". Sweden started to make the change from the Julian calendar and towards the Gregorian calendar in 1700, but it was decided to make the (then 11-day) adjustment gradually, by excluding the leap days from each of 11 successive leap years, 1700 to 1740. Thus the Swedish calendar was be out of step with both the Julian calendar and the Gregorian calendar for 40 years.