A concurrent was the weekday of 24 March in the
Julian calendar counted from 1 to 7, regarding 1 as Sunday. It was used to calculate the Julian Easter during the Middle Ages. It was derived from the weekday of the first day of the
Alexandrian calendar during the 4th century, 1
Thoth(29–30 August), counting Wednesday as 1 (see
Planetary hours#History). Therefore, the following 5 Thoth was a Sunday and the following 28
Phamenoth(24 March Julian) [= 208 Thoth≡ 5 Thoth mod 7] was also a Sunday.[1][2] It was first mentioned by
Dionysius Exiguus in 525 in his Latin version of the original Alexandrian Church's Greek
computus.[3] The insertion of the sixth
epagomenal day (29 August Julian) immediately before 1 Thoth was compensated for by the
bissextile day(24 February Julian) inserted six months later into the Julian calendar.
The widely used post-Bedan solar cycle (first year 776), which repeats every 28years, had concurrents of
1 2 3 4 6 7 1
2 4 5 6 7 2 3
4 5 7 1 2 3 5
6 7 1 3 4 5 6.
It skips a concurrent every four years due to a bissextile day in the Julian calendar a month earlier.[4][5] The Sunday after the next Luna 14 was Easter Sunday (see
Computus#Julian calendar). The concurrent is not used by the Gregorian Easter.
References
^Neugebauer, Otto (2016) [1979], Ethiopic Astronomy and Computus (Red Sea Press ed.), Red Sea Press, p. 58,
ISBN978-1-56902-440-9. The pages in this edition have numbers six less than the corresponding pages in the original edition, so six must be subtracted from most internal page references in this edition's index, etc.