Growing up in USSR, since I was a kid, I remember I always had a confusion regarding the calendar. While most dates were normal, a few were referred to as “old style” dates. And even though I’ve asked around and it was explained to me a few times, I never truly understood what it was about. I just new there was some calendar change in the past that created the shift.
Today I tweeted about Russia celebrating the Unity Day. Which I think is a silly replacement holiday for a huge celebration of the October Revolution. For as long as I remember, it was celebrated on November 7th. Then I realized that celebrating an October Revolution in November sounds strange. Then I remembered that the revolution actually took place on October 25th, the “old style”. And November 7th is the same date but the “new style”.
I am not a little boy back in the USSR anymore, I thought. I am a man in the modern age, equipped with powerful tools, such as Google and Wikipedia. No more should I suffer the confusion. And thus I quickly found out the source and history of “old style” and “new style”. Here is the relevant snipped from the Wikipedia page on the migration from Julian to Gregorian calendar. I’ve highlighted the important and interesting bits.
The Julian calendar was in general use in Europe and Northern Africa from the times of the Roman Empire until 1582, when Pope Gregory XIII promulgated the Gregorian calendar. Reform was required because too many leap days are added with respect to the astronomical seasons on the Julian scheme. On average, the astronomical solstices and the equinoxes advance by about 11 minutes per year against the Julian year. As a result, the calculated date of Easter gradually moved out of phase with the March equinox. While Hipparchus and presumably Sosigenes were aware of the discrepancy, although not of its correct value, it was evidently felt to be of little importance at the time of the Julian reform. However, it accumulated significantly over time: the Julian calendar gained a day about every 134 years. By 1582, it was ten days out of alignment from where it supposedly was in 325 during the Council of Nicaea.
The Gregorian calendar was soon adopted by most Catholic countries (e.g. Spain, Portugal, Poland, most of Italy).
Protestant countries followed later, and the countries of Eastern Europe adopted the “new calendar” even later. In the British Empire (including the American colonies), Wednesday 2 September 1752 was followed by Thursday 14 September 1752. For 12 years from 1700 Sweden used a modified Julian calendar, and adopted the Gregorian calendar in 1753, but Russia remained on the Julian calendar until 1918 (1 February became 14 February), after the Russian Revolution (which is thus called the “October Revolution” though it occurred in Gregorian November), while Greece continued to use it until 1924.