Etymology
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Santa Claus (n.)

1773 (as St. A Claus, in "New York Gazette"), American English, in reference to the customs of the old Dutch colony of New York, from dialectal Dutch Sante Klaas, from Middle Dutch Sinter Niklaas "Saint Nicholas," bishop of Asia Minor who became a patron saint for children. Now a worldwide phenomenon (Japanese santakurosu). Father Christmas is attested from 1650s.

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blue moon (n.)

"a long time," 1821, often in phrases indicating something rarely occurring. Compare at the Greek calends (from an ancient Roman phrase alluding to the fact that the Greeks had nothing corresponding to the Roman calends), and the native in the reign of Queen Dick and Saint Geoffrey's Day "Never, there being no saint of that name," reported in Grose (1788). Nevermass "date which never comes" is from 1540s. Blue moon is suggested earliest in this couplet from 1528:

Yf they say the mone is blewe,
We must beleve that it is true.

Though this might refer to calendrical calculations by the Church. Thus the general "rareness" sense of the term is difficult to disentangle from the specific calendrical one (commonly misinterpreted as "second full moon in a calendar month," but actually a quarterly calculation). In either case, the sense of blue here is obscure. Literal blue moons do sometimes occur under extreme atmospheric conditions.

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