On 24 February 1582, Pope Gregory XIII issued a papal bull or charter that began with the words Inter gravissimas ¹Translation: ‘Among the gravest…’ to reform the Julian calendar then in common use in Europe so that the seasonal events that were used to calculate Easter would be back in their proper places.

The Julian calendar year was 365 days, 6 hours long, however the average length of a year is 365 days, 5 hours and 49 minutes, an 11 minute difference.

In 325 CE, the council of Nicaea had established that Easter should be observed on the first Sunday after the first full moon on or after the first day of spring, which at the time, coincided with the vernal equinox. However over the intervening years, the 11 minutes had accumulated, and the first day of spring and the time of the vernal equinox were separated by ten full days. ²The vernal equinox is the time of the year when the Sun crosses the celestial equator and the length of the day and night is nearly equal.

Accordingly, on 24 February 1582, the Pope Gregory XIII decreed that the countries under his church should skip ten days to get back in sync. Spain, parts of Italy, the Netherlands, France, Portugal, Luxembourg, Poland and Lithuania all complied and 4 October 1582 was followed by 15 October 1582. The removal of the ten days from the calendar brought the vernal equinox back in line with the first day of spring, March 21.

Over the next 50 years, Austria, Switzerland, Germany, Hungary, and Prussia also fell into line, leaving Britain as one of the few holdouts. Finally in 1752, with the dual dating system across Europe causing too much confusion, Britain finally decided to acquiesce. There had been some opposition to the change, as there always is. The Church was against the change and petitioned the Crown not to adopt it.

Finally, after much debate, the changes were put into a Bill and brought to Parliament. As well as removing what now amounted to eleven days, the bill would change the first day of the year from 25 March to 1 January.

And so it was that all of Britain went to sleep on 2 September 1752 and awoke the next morning on 14 September 1752 and the eleven days became ghost days.

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Not everyone readily accepted the new calendar or the missing eleven days, with some villages simply refusing to recognise the change. On the first Christmas of the Gregorian calendar, many simply ignored the day on 25 December and waited another eleven days for Christmas.

With Britain’s adoption, most of Europe was using the Gregorian calendar, but there were still a few that refused to adopt it. Russia did not make the change until 1918 and Greece waited even longer, finally making the switch in 1923.