Regina Caeli (Queen of Heaven) Explained - Prayer that Replaces the Angelus at Easter - Legend Says Pope Gregory Heard Angels Pray this to End a Pestilence in Rome!

Regina Caeli (Queen of Heaven) Prayer.
This prayer honoring the mother of Jesus, is said in place of the Angelus, during the Easter season, at 6am, 12pm and 6pm daily. It is also the Marian antiphon that ends Compline (Night Prayer) in this season. It can be traced back to the 12th century.
The author is unknown. Tradition says that Pope St Gregory the Great (who lived from 590−604) heard angels chanting the first three lines one Easter morning and was inspired to add the fourth line.  

 This was during a procession with an image of the Blessed Virgin that was held to pray for the ending of a pestilence in Rome.

WORDS (FULL TEXT) OF THE PRAYER in Latin
Regina cæli, lætare, alleluia:
R. Quia quem meruisti portare, alleluia,
Resurrexit, sicut dixit, alleluia,
R. Ora pro nobis Deum, alleluia.
Gaude et lætare, Virgo Maria, alleluia.
R. Quia surrexit Dominus vere, alleluia.

Oremus.
Deus, qui per resurrectionem Filii tui, Domini nostri Iesu Christi,
mundum lætificare dignatus es:
præsta, quæsumus, ut per eius Genitricem Virginem Mariam,
perpetuæ capiamus gaudia vitæ.
Per eundem Christum Dominum nostrum. R. Amen.

ENGLISH TRANSLATION
Queen of Heaven, rejoice, alleluia.
The Son whom you merited to bear, alleluia.
Has risen, as He said, alleluia.
Pray for us to God, alleluia.
V. Rejoice and be glad, O Virgin Mary, alleluia.
R. For the Lord has truly risen, alleluia.
Let us pray.
O God, who through the resurrection of your Son Jesus Christ
gave rejoicing to the world,
grant, we pray, that through his Mother, the Virgin Mary,
we may obtain the joy of everlasting life.
Through Christ our Lord. Amen.