CONCURRENT NUMBERS. - Conventional figures used by computists to determine on which day of the week the beginning of each year falls, and thus to establish when Easter occurs.
The concurrent number, also called the "solar epact" or "major epact," for a given year indicates how many days have elapsed between the last Sunday of the previous year and the beginning of the new year. Since Easter is regulated by the spring equinox, the relationship is calculated with respect to March 24: considering that from the beginning of the year to this date there are 82 days (83 in leap years), i.e., 11 weeks plus 5 days, March 24 will fall on a day of the week that is shifted by 5 units from that of January 10; that is, if in a given year January 10 falls on a Sunday, March 24 will fall on a Friday, and so on. And since the dominical letters (v. DOMINICAL LETTER) correspond to the concurrent numbers as follows: A=6, B=5, C=4, D=3, E=2, F=1, G=7, the following relationship exists between the concurrent numbers and the day of the week on which March 24 falls:
| Concurrent Numbers | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| dominical letter | F | E | D | C | B | A | G |
| day of the week on Jan. 10 | Tues. | Wed. | Thurs. | Fri. | Sat. | Sun. | Mon. |
| and on Mar. 24 | Sun. | Mon. | Tues. | Wed. | Thurs. | Fri. | Sat. |
In leap years, the concurrent number naturally shifts by one unit on February 29.