solemnity

Short definition from Wiktionary

Noun

1. 1
   The quality of being deeply serious and sober or solemn.
2. 2
   An instance or example of solemn behavior; a rite or ceremony performed with reverence.
3. 3
   A feast day of the highest rank celebrating a mystery of faith such as the Trinity, an event in the life of Jesus, the Virgin Mary, or another important saint.
4. 4
   A solemn or formal observance; proceeding according to due form; the formality which is necessary to render a thing done valid.
5. 5
   A celebration or festivity.