Thirty-one is prime, and its principal traditional weight is again a matter of gematria: the Hebrew word El, one of the oldest names of God, is spelled aleph and lamed, one and thirty, totalling thirty-one. The name is the common Semitic word for a god and forms the root of Elohim and of countless personal names, Isra-el, Micha-el, Gabri-el, so its numerical value of thirty-one carries the weight of the divine name in the shorter form. As with twenty-six for the Tetragrammaton and eighteen for chai, the significance is generated by the letters of a word rather than by counting, and thirty-one belongs to the small set of numbers that Jewish tradition treats as sacred because a name of God resolves to them.
Cheiro's 1926 Book of Numbers treats thirty-one as sharing the same reading as thirty-two, a step in the compound-number scheme where several consecutive figures are read together rather than individually; Florence Campbell's 1931 book and Juno Jordan's 1965 systematisation both preserved the pairing. Outside numerology, thirty-one has a wholly secular modern association: Baskin-Robbins, founded in Glendale, California in 1953, built its identity around the slogan "31 flavors," one flavor for every day of the month, an American commercial coinage with no bearing on El or on any older tradition.