“The year hath twelve months, comprising fifty-two weeks, and one day : indeed, it has three hundred and sixty-five days, and nearly six hours ; for in such an interval of time the sun traverses the zodiac. However, in the course of four years, four times six hours constitutes an extra day ; hence that year is called intercalary, bissextus, or bissextile.”

– De anno et ejus partibus (my working translation)

“In a bissextile year, the month of February is 29 days long, and the feast of St. Matthias [note: usually on the 24th] is celebrated on the 25th day of February, and the feast of St. Gabriel of Our Lady of Sorrows [note: usually on the 27th] on the 28th day of February, and the Sixth Kalends are said twice, that is, on the 24th day and on the 25th day ; and the dominical letter, which was taken up in the month of January, is changed into the preceding one : as for example, if in January the dominical letter was A, it is changed into the preceding one, which is g, etc., and the letter f serves twice, on the 24th and on the 25th.”

– Calendarium romanum (my working translation)