"This Sunday, called, from the first word of the Introit, Laetare Sunday, is one of the most solemn of the year. The Church interrupts her Lenten mournfulness; the chants of the Mass speak of nothing but joy and consolation; the Organ, which has been silent during the preceding three Sundays, now gives forth its melodious voice; the Deacon resumes his Dalmatic, and the Subdeacon his Tunic; and instead of purple, Rose-coloured Vestments are allowed to be used. These same rites were practised in Advent, on the third Sunday, called Gaudete. The Church’s motive for introducing this expression of joy in to-day’s Liturgy, is to encourage her Children to persevere fervently to the end of this holy Season. The real Mid-Lent was last Thursday, as we have already observed; but the Church, fearing lest the joy might lead to some infringement on the spirit of penance, has deferred her own notice of it to this Sunday, when she not only permits, but even bids, her children to rejoice!"
-The Liturgical Year, Dom Prosper Guéranger