What is Solemnity?

Legal Definition
A solemnity is a feast day of the highest rank in the liturgical calendar of the Roman Rite, celebrating a mystery of faith such as the Trinity, an event in the life of Jesus, his mother Mary, or another important saint. The observance begins with the vigil on the evening before the actual date of the feast. Unlike feast days of the rank of feast (other than feasts of the Lord) or those of the rank of memorial, solemnities replace the celebration of Sundays outside of Advent, Lent and Easter (those in Ordinary Time).

The word comes from Latin sollemnitas, derived from sollus (whole) and annus (year), indicating an annual celebration.
-- Wikipedia
Legal Definition
The formality established by law to render a contract, agreement, or other act valid.

2. A marriage, for example, would not be valid if made in jest, and without solemnity. Vide Marriage, and Dig. 4, 1, 7; Id. 45, 1, 30.
-- Bouviers Law Dictionary
Legal Definition
A rite or ceremony; the formality established by law to render a contract, agreement or other act valid.
-- Black's Law Dictionary