When you are looking through the list of available local calendars in Universalis, you may be puzzled by the mysterious “Eastern General” calendar which appears just before “Europe” in the list. Here is an explanation.

The Orthodox Church, like the Anglican Church, rejected the scientifically-based reform to the calendar introduced by Pope Gregory XIII in 1582 after hundreds of years of astronomical research. The Anglicans, and Protestants generally, moved to the Gregorian Calendar a century or two later, but the Orthodox never did. Accordingly dates in the Orthodox calendar happen 13 days later than their homologues in our calendar: their New Year is celebrated on our 14 January and their Christmas on our 7 January. This inevitably has consequences for the date of Easter. Easter is the Sunday after the first full moon after 21 March, so if the sequence in any given year is “our 21 March, the full moon, their 21 March (our 3 April)” then the Eastern Easter will be one moon later than ours: in other words, about a month later. (There is actually more scope for variation since the Pope also updated the algorithms for calculating the date of the full moon, another change that the Orthodox didn’t follow).

For Catholics living in Orthodox lands, the result of following the usual Catholic calendar would be that their Easter (and Lent, and Pentecost,…) was a week or a month out of step with their neighbours’. For the sake of harmony, they therefore use a hybrid calendar in which the dates of feasts and saints are the same as ours – Christmas on our 25 December, not on our 3 January – but Easter is celebrated on the same day the Orthodox celebrate it.

And that is what the “Eastern General” calendar in Universalis does.