Burning of the Clavie on New-year's eve (old style) in the village of Burghead, on the southern shore of the Moray Frith. The clavie is a sort of bonfire made of casks split up. One of the casks is split into two parts of different sizes, and an important item of the ceremony is to join these parts together with a huge nail made for the purpose. Whence the name clavus (Latin), a nail. Chambers, who in his Book of Days (vol. ii. p. 789) minutely describes the ceremony, suggests that it is a relic of Druid worship, but it seems to me to be connected with the Roman ceremony observed on the 13th September, and called the clavus annalis. The two divisions of the cask, I think, symbolise the old and the new year, which are joined together by a nail. The two parts are unequal, because the part of the new year joined on to the old is very small in comparison.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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