SOLEMN (Lat. sollemnis, sollennis, less correctly solennis, yearly, annual; from sollus=totus, whole, entire, Gr. oXos, and annus, year), properly that which occurs annually, hence at stated intervals, regular, established; the term being particularly used of religious rites or ceremonies which recur at stated intervals, hence festive, sacred, marked by religious ceremony or ritual, and so grave, impressive, serious, the most general current usage. Another branch of meaning stresses the formal, customary aspect; and hence in such phrases as " solemn act," probate in " solemn form," it means that which is done with all due forms and ceremonies.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)