Solemnities are feast days that are a little something special. The greatest saints are celebrated on solemnities, and the importance of these days is reflected in the law of the Church. When a solemnity falls on a Friday, the Lenten abstinence from meat is overridden.
It is a pious practice, rather than a reflection of Church law, but it is also traditional to relax your Lenten penance on a solemnity such as this. The penances of Lent pause for a celebration of God’s goodness.