The following paragraphs about the dates of Easter relating to the dates of Septuagesima, which we will observe this coming Sunday, were sent to me by a friend. The author is Joshua , a Roman Catholic who attends Mass in its Traditional form, in Latin, where a lectionary very close to our own is observed. He wrote these paragraphs and put them on his blog Psallite Sapienter. I found the quite interesting.

The Keys of Septuagesima

A pleasant coincidence, to which my attention was drawn while reading Dom Gueranger:

"As the season of Septuagesima depends upon the time of the Easter celebration, it comes sooner or later according to the changes of that great feast. January 18 and February 22 are called the 'Septuagesima keys', because the Sunday, which is called Septuagesima, cannot be earlier in the year than the first, nor later than the second, of these two days."

In other words, Septuagesima can fall as early as the 18th of January - formerly, one of the two feasts of the Chair of St Peter (and still able to be celebrated according to the '62 Missal) - or as late as the 22nd of February, the other (remaining) feast of the Chair of St Peter.

Hence, these two dates are called the "keys" of Septuagesima.

Septuagesima can only fall on the 18th of January when Easter falls on its earliest possible date, the 22nd of March, in an ordinary, non-leap year: this last happened in 1818 and will next occur in 2285.

But as for Septuagesima falling on the 22nd of February - that can only happen when Easter falls on its latest possible date, the 25th of April, AND in a year that is a leap year. This has not happened since Pope Gregory brought in his new calendar and new system of calculating Easter, nor will it happen until some time after 3000AD. (Easter last fell as late as this in 1943, and this will happen again in 2038, but neither years are leap years, so Septuagesima in both cases takes place on the 21st of February.)

Apparently, the last time Septuagesima did occur on the 22nd of February happened under the old Julian calendar, according to the system then used to compute the date of Easter - in 1204. (And before that, in 672 according to the Roman method - but at that time several competing systems for finding the date of Easter were still in competition, as exemplified by the disputes leading to the famous Synod of Whitby.)

[For those parts of Protestant Europe that were still using the Julian calendar, in sectarian opposition to the Gregorian reform, 1736 was a year in which Septuagesima fell on the 22nd of February - this would have included Anglican England and Lutheran Scandinavia and Germany. The so-called Dionysian method for finding Easter, together with the Julian calendar, has Septuagesima on the 22nd of February quite punctually every 532 years; but this method has been long abandoned in the West, and of course the East doesn't keep Septuagesima.]

*Update* - after trawling through results from online Easter calculators, I find that the next bissextile (that is, leap) year to come in which Easter falls on the 25th of April is - 3784. So, provided the calendar is not amended (the Gregorian calendar would have lost about a day by then), nearly 18 centuries hence, and over 2500 years since it last happened, Septuagesima will fall on its latest possible date of the 22nd of February, in 3784.

(BTW, in 2011, only three years from now, Easter falls on the 24th of April, its second-latest possible day, so Septuagesima will then fall on the 20th of February, rather than on the 20th of January as it has done this year, (2008) in which Easter falls on its second-earliest possible date.)