Christos Anesti (Christ Has Risen) is the traditional Easter greeting in the Greek Orthodox Church. You may wonder why are we writing about Easter a week late, but we aren’t — at least for the Orthodox Christians in the world who use the Julian calendar for the dates of all their holy days. What is the Julian calendar? Click the video and listen to the hypnotic Easter chant, said many times during the midnight services and brush up on your knowledge of horology (the study of measurements of time).

The Julian calendar, proposed by — you guessed it, Julius Caesar — was a reform of the Roman calendar. The Roman calendar consisted of 10 months beginning in spring with March; winter was left as an unassigned span of days. These months ran for 38 nundinal cycles, roughly an eight-day week ended by religious rituals and a public market. The Julian calendar took effect on 1 January 45 BC. It was the predominant calendar for the western world until it was refined and gradually replaced by the Gregorian calendar, the one we use today.

The Julian calendar gains against the mean tropical, or the time that the sun takes to return to the same position in the cycle of seasons at the rate of one day in 128 years. The Gregorian calendar it shifts one day in 3,030 years. The difference in the average length of the year between Julian (365.25 days) and Gregorian (365.2425 days) is 0.002%. You could think of it like a mechanical watch that loses a minute each year so that in 60 years, the watch is an hour behind. The Gregorian calendar makes adjustments along the way; the Julian doesn’t.

I know that is a bit confusing. The Julian calendar has a 365-day year divided into 12 months. The calendar adds a leap day to February every four years. So the Julian year is 365.25 days long. When you add all this up the calendar adds three days every four centuries relative to the astronomical equinox. This discrepancy was corrected by the Gregorian reform in 1582; they attempted to compensate for the time slippage. The Gregorian calendar has the same months and month lengths as the Julian calendar, but, in the Gregorian calendar, years evenly divisible by 100 are not leap years unless they are evenly divisible by 400. The result of this is as of February 16, 1900 on the Julian calendar and March 1, 1900 on the Gregorian calendar, the Julian calendar is 13 days behind. The Eastern Orthodox Church, which the Greek Orthodox Church is a part of still uses the Julian calendar for calculating the date of Easter. So the date differs from year to year, sometimes matching up with the Western Church. A good way to remember is that in the Orthodox Church Easter is always after Passover.

The most important thing about all of this is to remember that children that have a parent from the Eastern Church and the Western Church on years when Easter is on different dates, should expect two Easter baskets and twice the candy.