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]. —
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.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894