Normally, the Solemnity of the Annunciation, the celebration of the angel visiting Mary and asking her if she would bear Jesus, is on March 25th. Liturgical pregnancies are always nine months long (March 25th to December 25th, or the birth of Mary is celebrated nine months after the Immaculate Conception).
This year, though, the celebration is transferred to April 9th. Why?
Calendar rubrics FTW. The vast majority of liturgical celebrations are simply not celebrated when a “higher ranking” feat takes place on the same day. The memorial of St. Ceallach in Ireland is usually celebrated on April 1st, but wasn’t celebrated at all since that was Easter Sunday. Solemnities, however, are transferred forward to the next available date that does not outrank it.
In 2018, March 25th fell on Palm Sunday, so the Annunciation would be pushed forward to Monday, March 26th. All of Holy Week, however, outranks the Annunciation, so it would to be pushed forward to Sunday, April 1st, which being Easter obviously outranks. The entire octave of Easter (the eight days from Easter to Divine Mercy Sunday/the 2nd Sunday of Easter) share the rank of Easter, so the Annunciation had to keep moving forward. The Monday of the 2nd week of Easter is known as “low Monday” from the Extraordinary Form where a “low Mass” would be celebrated for the first time in awhile. Be it a regular weekday of Easter, the solemnity outranks it and Mary’s visit from an angel found a home for 2018.
For your own edification, the Table of Liturgical Days is available from the Order of St. Benedict site. The table originated in Pope Paul VI’s Universal Norms on the Liturgical Year and the Calendar which is what guides local bishop conferences and how all those calendar printers get things right.