From the sermon linked below:
Four words perfectly describe the feast of the Immaculate Conception: “Potuit, Decuit, ergo fecit–He could, He should, therefore He did it.”
Potuit (He could) – God is all powerful, and therefore could have granted the singular grace of the Immaculate Conception.
Decuit (He should) – It is only fitting that Mary should be preserved from any stain of sin and the grace to never fall under the dominion of the devil, so as to be worthy to be the mother of God.
Ergo fecit (Therefore he did it) – Since God could create Our Lady spotless from the first moment of her conception, and since he should do this, he did.
Happy Feast Day Everyone!