The Enoch calendar is an ancient calendar described in the pseudepigraphalBook of Enoch. It divided the year into four seasons of exactly 13 weeks. Each season consisted of two 30-day months followed by one 31-day month, with the 31st day ending the season, so that Enoch's year consisted of exactly 364 days.

The Enoch calendar was purportedly given to Enoch by the angelUriel. Four named days, inserted as the 31st day of every third month, were named instead of numbered, which "placed them outside the numbering". The Book of Enoch gives the count of 2,912 days for 8 years, which divides out to exactly 364 days per year. This specifically excludes any periodic intercalations.

Evaluation

Calendar expert John Pratt wrote that "The Enoch calendar has been criticized as hopelessly primitive because, with only 364 days, it would get out of sync with the seasons so quickly: in only 25 years the seasons would arrive an entire month early. Such a gross discrepancy, however, merely indicates that the method of intercalation has been omitted."^{[1]} Pratt pointed out that by adding an extra week at the end of every seventh year (or Sabbatical year), and then adding an additional extra week to every fourth Sabbatical year (or every 28 years), the calendar could be as accurate as the Julian calendar.

^John Pratt, Review of Mapping Time:, The Calendar and Its History, by E. G. Richards, American Mathematical Monthly, volume 107, number 1, January, 2000

Further reading

See the various writings of Julian Morgenstern, James C. VanderKam and others.

"Sabbatical Years and the Year of Jubilee". Sidney B. Hoenig; The Jewish Quarterly Review, New Series, Vol. 59, No. 3 (Jan., 1969), pp. 222–236.

"A Possible Method of Intercalation for the Calendar of the Book of Jubilees". E. R. Leach; Vetus Testamentum, Vol. 7, Fasc. 4 (Oct., 1957), pp. 392–397.

"Studies in the Hebrew Calendar: (Interpretation of a Difficult Passage in the Palestinian Talmud)". Solomon Gandz. Proceedings of the American Academy for Jewish Research, Vol. 17, (1947-1948), pp. 9–17.

"Chronology of the Account of the Flood in P.--A Contribution to the History of the Jewish Calendar. Benjamin Wisner Bacon. Hebraica, Vol. 8, No. 1/2 (Oct., 1891 - Jan., 1892), pp. 79-88.

"The Calendar of the Book of Jubilees, Its Origin and its Character." Julian Morgenstern. Vetus Testamentum, Vol. 5, Fasc. 1 (Jan., 1955) pp. 34–76.

"The Judean Calendar during the Second Commonwealth and the Scrolls." Solomon Zeitlin. The Jewish Quarterly Review. New Series, Vol. 57, No. 1 (Jul., 1966), pp. 28–45.