Habitual, customary. (Latin, sollemnis, strictly speaking means “once a year,” “annual,” solus-annus.)
“Silent night with this her solemn bird” [i.e. the nightingale, the bird familiar to night]. —Milton: Paradise Lost, v.
Of course the usual meaning of “solemn” is devout; but an annual festival, like Good Friday, etc., may be both devout and serious. The Latin for “it is usual,” is solemne est, and to “solemnise” is to celebrate an annual custom.
The Solemn Doctor. Henry Goethals was so called by the Sorbonne. (1227-1293.) Solemn League and Covenant, for the suppression of Popery and Prelacy, adopted by the Scotch Parliament in 1638, and accepted by the English in 1643. Charles II. swore to the Scotch that he would abide by it and therefore they crowned him in 1651 at Dunbar; but at the Restoration he not only rejected the covenant, but had it burnt by the common hangman.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894