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Now, to business.
Each Mass has a set of what are called Propers. Some have very special Propers, such as the feast of St. Therese of Lisieux, or the First Sunday of Advent. Others have more general Propers, such as St. Thomas Aquinas or St. Jerome. Occasionally the Mass has no assigned saint, and we celebrate either a "feria", for which see the previous Sunday, or a votive Mass, for which see the Votive Mass in question.
Another way to answer the question is this: consult the altar Missal. Missalettes (in pews) are usually helpful, so long as you don't want music for the introit in question.
Still a further way to answer the question is this: there are several books of Proper chants, the best known of which is called the Liber Usualis, which is organized into several sections according to season and according to feast days of saints. These will have music.
EF ? Easiest is Liber Usualis, as above. Available on this Musica Sacra website under Resources. OF in Latin? 1/ With the current edition of Ordo Cantus Missae in one hand, Here. And the Graduale Romanum in the other hand, on this Musica Sacra site on the Resources page. OR 2/ with the Graduale Simplex, also available here on the Resources page.
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