Straight from Catholic Answers:
There are thus four options for the entrance song:
- the entrance antiphon and psalm of the âRoman Gradualâ;
- the entrance antiphon and psalm of the âSimple Gradualâ;
- song from other collections of psalms and antiphons;
- other sacred song chosen in accord with the above criterion.
The entrance antiphon is a seasonal (e.g. Christmas, Easter, Ordinary Time) verse from the Bible accompanied by a psalm. On Sunday Masses, the entrance hymn is usually linked to the prescribed entrance antiphon and psalm of the day. Thus, you would not hear âWe Three Kingsâ sung as the entrance antiphon on Palm Sunday. Usually the entrance antiphon (hymn) on Sundays is going to have something to do with the written entrance antiphon and psalm. In daily Masses where there is no singing the entrance antiphon and psalm are recited as in 1 and 2 above.