Outside of the recitation of the Creed on the Annunciation and Christmas, there are a few times when the entire congregation would once genuflect during a reading or proclamation at Mass.
One of those times was during the Epistle (now called the Second Reading) at Mass on Palm Sunday.
It is customary to proclaim the Letter of St. Paul to the Philippians, where he writes about Jesus' sacrifice on the cross.
Dom Prosper Gueranger explains in the Liturgical Year why Catholics would genuflect during this reading on Palm Sunday.
While this custom is not included in today's Roman Missal, it remains a beautiful tradition that is preserved by the Extraordinary Form of the Mass.