Fourth Day - Fiat mihi secundum verbum tuum ("Be it done unto me according to Thy word.") Dixit autem Maria:... fiat mihi secundum verbum tuum. Et discessit ab illa angelus. (St. Luke i, 38: "And Mary said: ... be it done to me according to thy word. And the angel departed from her.")
The adorable Heart of Jesus Christ began to beat with a love at once human and divine after the Virgin Mary generously pronounced Her "Fiat"
Pius XII, Haurietis Aquas
...just as Christ, because He redeemed us, is our Lord and king by a special title, so the Blessed Virgin also (is our queen), on account of the unique manner in which she assisted in our redemption, by giving of her own substance, by freely offering Him for us, by her singular desire and petition for, and active interest in, our salvation.
From these considerations, the proof develops on these lines: if Mary, in taking an active part in the work of salvation, was, by God's design, associated with Jesus Christ, the source of salvation itself, in a manner comparable to that in which Eve was associated with Adam, the source of death, so that it may be stated that the work of our salvation was accomplished by a kind of "recapitulation," in which a virgin was instrumental in the salvation of the human race, just as a virgin had been closely associated with its death; if, moreover, it can likewise be stated that this glorious Lady had been chosen Mother of Christ "in order that she might become a partner in the redemption of the human race"; and if, in truth, "it was she who, free of the stain of actual and original sin, and ever most closely bound to her Son, on Golgotha offered that Son to the Eternal Father together with the complete sacrifice of her maternal rights and maternal love, like a new Eve, for all the sons of Adam, stained as they were by his lamentable fall," then it may be legitimately concluded that as Christ, the new Adam, must be called a King not merely because He is Son of God, but also because He is our Redeemer, so, analogously, the Most Blessed Virgin is queen not only because she is Mother of God, but also because, as the new Eve, she was associated with the new Adam.
Pius XII, Ad Cæli Reginam