Most Catholics today are used to the Mass being conducted in English, and they hardly ever think about the fact that Latin remains the official language of the Catholic Church. But occasionally, Latin reasserts itself, as it does in the case of Laetare Sunday, the popular name for the Fourth Sunday in Lent. Laetare means "Rejoice" in Latin, and the Introit (entrance antiphon) in both the Traditional Latin Mass and the Novus Ordo is Isaiah 66:10-11, which begins "Laetare, Jerusalem" ("Rejoice, O Jerusalem").
Because the midpoint of Lent is the Thursday of the third week of Lent, Laetare Sunday has traditionally been viewed as a day of celebration, on which the austerity of Lent is briefly lessened. The passage from Isaiah continues, "rejoice with joy, you that have been in sorrow," and on Laetare Sunday, the purple vestments and altar cloths of Lent are set aside, and rose ones are used instead. Flowers, which are normally forbidden during Lent, may be placed on the altar. Traditionally, the organ was never played during Lent, except on Laetare Sunday.
Laetare Sunday is also known as Rose Sunday, sand it has a counterpart in Advent: Gaudete Sunday, the Third Sunday of Advent, when purple vestments are exchanged for rose ones. The point of both days is to provide us encouragement as we progress toward the end of each respective penitential season.