REGULARS. — Conventional and invariable numbers used to determine certain dates; there were three kinds of r., namely the solar r. (regulares solis), the lunar r. (regulares lunae), and the Easter r. (regulares Paschae).
Added to the concurrent number of the year, the solar r., which numbered twelve, gave the weekday of the first day of the month; the lunar r., on the other hand, were used to determine the age of the moon on the first day of each month, according to the formula: epact − lunar r. = age of the moon; the Easter r., finally, numbering seven and corresponding to the weekday letters (v. LETTERA DOMENICALE), were used to determine on which day the xiv of the March moon fell, that is, the so-called terminus paschalis, necessary for establishing the date of Easter.