Normally, on March 25th we celebrate the Feast of the Annunication of the Lord- where we remember the archangel Gabriel's visit to Our Lady to 'announce' that she would conceive Jesus, and celebrate Mary's "yes" to God's plan. As the celebration of Jesus' conception in Mary's womb, it makes sense to celebrate nine months before Christmas.
This year, however, March 25th falls during Holy Week- and the days of Holy Week and the Sacred Triduum outrank all the other days of the liturgical year. When the Annunciation falls during Holy Week, it is moved to the next available day. Since it cannot be celebrated on the days of Holy Week nor the days of the week after Easter (called the Octave of Easter), it is moved to the Monday following the Second Sunday of Easter, which is April 8th this year.
The Annunciation is the patronal feast of our Diocese - be sure to celebrate!