What is the difference between the Julian and Gregorian calendar?
Quick Answer: The Julian calendar, introduced by Julius Caesar, adds a leap day every 4 years, making its year 365.25 days long. The Gregorian calendar, introduced in 1582, refines this by skipping some leap years to better match the Earth's orbit, making the average year 365.2425 days. This fixes the drift of dates over centuries.
The Julian calendar assumed a year was exactly 365.25 days, but the actual solar year is slightly shorter. Over centuries, this caused calendar dates to drift away from the seasons. To fix this, Pope Gregory XIII introduced the Gregorian calendar, which skips leap years on century years not divisible by 400. This keeps the calendar more aligned with the Earth's orbit.
When the Gregorian calendar started, 10 days were dropped to realign dates with the seasons. Today, most of the world uses the Gregorian calendar for its improved accuracy. Some Orthodox churches still use the Julian calendar for religious holidays, so the two calendars can show different dates for the same event.