Laetare Sunday

The 4th Sunday of Lent is known as Laetare Sunday, because the first word of the Introit (ie. Entrance Antiphon) is "Laetare". The vestments used by the priest can be pink, instead of violet.

Laetáre Ierúsalem, et convéntum fácite omnes qui dilígitis eam: gaudéte cum laetítia, qui in tristítia fuístis: ut exsultétis, et satiémini ab ubéribus consolatiónis vestrae.

Laetátus sum in his quae dicta sunt mihi: in domum Dómini íbimus.

Rejoice, O Jerusalem; and gather round, all you who love her; rejoice in gladness, after having been in sorrow; exsult and be replenished with the consolation flowing from her motherly bosom. (Isaiah 66:10-11)

I rejoiced when it was said unto me, "Let us to go the house of the Lord". (Psalm 122(121): 1)

Translation from The Gregorian Missal.

It signifies a time of restrained rejoicing, which I personally observe is manifested in the words of the opening prayer.

Father of peace, we are joyful in your Word, your Son Jesus Christ, who reconciles us to you. Let us hasten towards Easter with the eagerness of faith and love.

More from the Catholic Encyclopedia also. Fr John Zulhsdorf also explains a bit and provides a more literal translation of the Latin of the Opening prayer.

I hope the priest celebrating Mass in your parish mentions the why and what of Laetare Sunday. Mine did, and I'm grateful.