Many people don’t realize that Holy Church considers some Old Testament figures to be saints with their own feast days. We don’t celebrate them at the altar, but they have a day.
Actually, if there were no other saint in the liturgical calendar taking precedence, we are told that we indeed could take a saint from the Martyrology. But I digress.
Today is the commemoration of the 8th c. St. Isaiah, the prophet of the Old Testament.
The Martyrologium Romanum has an entry for him.
1. Commemoratio sancti Isaiae, prophetae, qui, in diebus Oziae, Iotham, Achaz et Ezechiae, regum Iudae, missus est ut populo infideli et peccatori Dominum fidelem et salvatorem revelaret, ad implementum promissionis David a Deo iuratae. Apud Iudaeos sub Manasse rege martyr occubuisse traditur. … The commemoration of saint Isaiah, the prophet, who, in the days of Ozias, Jotham, Achaz and Ezechias, kings of Judah, was sent in order to reveal to the unfaithful and sinful People the faithful Lord, the Savior, for the fulfillment of the promise made by God to David. It is said that he sank into death among the Jews under King Manasse.