Britain once lost 11 days and moved New Year from March to January
In 1752 people in England, Wales, Ireland and the American colonies saw September lose 11 days as their calendar switched to the Gregorian system. The same year, the start of the legal year jumped from 25 March to 1 January. Scotland had made that second change back in 1600.
The problem was a small arithmetic error. The Julian calendar assumed a year lasts exactly 365.25 days, while the true figure is closer to 365.242. That meant too many leap years, and over the centuries the calculation of Easter, fixed in the 4th century, slipped out of line with the seasons. European countries adopted the Gregorian correction at different times between 1582 and 1923, and every extra century of delay added another day to skip. Britain needed to drop eleven days in 1752; when Russia changed its civil calendar in 1918, the gap had grown to thirteen.
Russia's switch came through a decree signed by Lenin, and the Julian dates from 1 to 13 February 1918 simply never happened: the day after 31 January was 14 February. For a few months the old date had to be written in brackets after the new one. That is why the October Revolution is often described as beginning on 25 October, or 7 November in the new reckoning.
The labels Old Style and New Style cover both kinds of change. From 1155 to 1752 the English civil year officially began on Lady Day, the Feast of the Annunciation, even though people had celebrated New Year on 1 January since at least the 13th century. Dates falling between January and late March are therefore the tricky ones, and to avoid confusion parish registers often wrote two year numbers for those months. Historians usually keep the day as originally recorded but adjust the year so it starts in January.
The split still shows at Christmas. Western churches and Eastern Orthodox churches both keep 25 December, but by different calendars, so their celebrations fall 13 days apart, a gap that holds until March 2100. Latin writers marked the difference with the abbreviations st.v. and st.n., for old and new style.
Source: Old Style and New Style dates
