Unlike western Europe, Imperial Russia did not use the revised calendar instituted by Pope Gregory XIII. Explain why the Russian Revolution, which started on November 7, 1917, according to the modern calendar, is called "the October revolution" in Russia. What was this date according to the Russian calendar at the time? Explain.

The Gregorian calendar, named after Pope Gregory XIII, who introduced it in 1582, is the most widely used calendar system today. Its primary purpose was to correct the 10-minute annual discrepancy of the Julian calendar by reforming the leap year system and realigning the date of the Easter celebration with the spring equinox. The key change involved skipping 10 days to correct the drift and adjusting the formula for leap years: every year that is exactly divisible by 4 is a leap year, except for years that are exactly divisible by 100, unless they are also divisible by 400.

For instance, the year 2000 was a leap year because it is divisible by 400, but 1900 was not because, while it is divisible by 100, it is not divisible by 400. This accuracy ensures that the Gregorian calendar remains more closely synced with the Earth's orbit around the Sun and the changing seasons. The adoption of the Gregorian calendar was gradual, with many countries transitioning from the Julian calendar at different times, leading to historical date discrepancies.

Before the Gregorian calendar took prominence, the Julian calendar was the standard for much of the world. Introduced by Julius Caesar in 45 BCE, it was a solar calendar with a year length of 365.25 days, divided into 12 months. A leap day was added every four years to account for the fractional day. However, this system slightly overstated the length of a year by about 11 minutes.

Over centuries, these minutes added up, and the calendar became out of step with the seasons. While the Gregorian reform corrected this by introducing a more precise leap year system, the Julian calendar remained in use in some parts of the world for a long time. This historical adherence to the Julian calendar, especially in places like Russia until after the October Revolution, had a profound impact on historical record-keeping and the naming of events such as revolutions.

The intersection of calendars and Russian history is most notably illustrated by the October Revolution. Due to Russia's continued use of the Julian calendar until January 1918, the revolution's name reflects the Julian dating of events — it began on October 25, 1917. This is about 13 days behind the Gregorian calendar date, which is November 7, 1917.

The label 'October Revolution' thus reflects the date according to the calendar in use in Russia at the time, despite the Gregorian calendar being adopted in many other parts of the world. It exemplifies how changes in calendar systems can lead to differences in historical interpretations and serves as a reminder of the complexities involved in studying past events from different cultural and chronological perspectives.