Roman Catechism 7.23

Chapter 7 — Part I: The Creed — Article V: He Descended into Hell; the Third Day He Rose Again from the Dead

Section 23. "The Third Day" — In explanation of the additional words of the Article, the third day, the pastor should inform the people that they must not think our Lord remained in the grave during the whole of these three days. But as He lay in the sepulchre one full day, a part of the preceding and a part of the following day, He is said, with strictest truth, to have lain in the grave for three days, and on the third day to have risen again from the dead.