Today is the (transferred) feast of the Annunciation: when the angel Gabriel appeared to Our Lady and told her that she would conceive of the Holy Spirit. This is the day on which "the Word became flesh and dwelt among us." Normally, we celebrate this feast on March 25 (because that is 9 months before Christmas). However, when March 25 takes place during Holy Week or the Paschal Octave, it is transferred to the Monday after the Paschal Octave. All of the antiphons sung during Paschaltide end with "alleluia" so these are the versions of the Propers with Alleluias in the Extraordinary Form. (All of them except the introit also work for the Latin Ordinary Form. The Introit in the Ordinary Form is different.)
Ant. All the rich among the people seek your face. Behind her, the virgins are brought to the King. They are borne into you with gladness and joy. Alleluia! Alleluia!
V. My heart overflows with a good word: I speak my ode to the King.
V. Glory be...
Ant. Hail, Mary, full of grace; the Lord is with thee. Blessed art thou amongst women, and blessed is the fruit of thy womb. Alleluia!
Ant. Behold! A Virgin shall conceive and bear a son: and shall call his name Emmanuel! Alleluia!