What was the first Latin word or expression used for Christmas, the Christian event in the honor of Jesus' birth? I know what to call Christmas in Latin, but it occurred to me that there is no guarantee that it wasn't called something entirely different back when the concept was first introduced in Latin. When you give attested mentions of Christmas, please also give an estimate for time, so that it is easier to put everything on a timeline.
I've found mentions of Christi nativitas (Christ's birth) in Tertullian and Cyprian (late 2nd to mid-3rd century), but Christmas was not widely celebrated before the 4th century (source), and it was called Christ's dies natalis. Augustine calls it dies natalis Domini nostri Iesu Christi, or simply dies natalis Domini. Later mentions of it, e.g. by Pope Leo, are similarly worded, though sometimes omitting dies—the origin of the Italian and Portuguese Natale/Natal. See the footnotes on this and the following page for citations.