Behind the tame Anglican title lies the Roman Catholic feast of the Immaculate Conception – meaning not the conception of Christ (which is our clear orthodox faith) but the conception of Mary herself in such a way as to render her sinless throughout her life (which has not commended itself to the universal church in the same way). The Anglican Collect below is therefore careful to make it clear that our focus remains on Christ, and steers away from the additional dogma.
The Immaculate Conception by Murillo
The idea that Mary as well as Christ remained sinless throughout her life after being miraculously made so ah her conception (we are dealing with Augustinian theology of original sin here, remember) gained ground in the middle ages (partly perhaps because of popular devotion and partly because high theologians could see it as a bulwark protecting the divinely incarnate nature of Christ himself).
We have heard something of papal Constitutions recently, and it was by one such in 1854 that Pope Pius IX promulged that Mary "in the first instance of her conception, by a singular privilege and grace granted by God, in view of the merits of Jesus Christ, the Saviour of the human race, was preserved exempt from all stain of original sin,” and that “this a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful"
Almighty and everlasting God,
who stooped to raise fallen humanity
through the child-bearing of blessèd Mary:
grant that we, who have seen your glory
revealed in our human nature
and your love made perfect in our weakness,
may daily be renewed in your image
and conformed to the pattern of your Son
Jesus Christ our Lord,
who is alive and reigns with you,
in the unity of the Holy Spirit,
one God, now and for ever.