This is not really a movable feast unless it falls on a Sunday or around Holy Week (for example, in 2018 it was celebrated on April 9th, after Divine Mercy Sunday, after Easter). This is because, while like all feasts it is secondary to the Easter Mysteries, it is a very important feast for us and must be celebrated and not subsumed - the day Mary said "yes", the day that changed hers, Joseph's, and our lives forever.
While we might struggle with the number of Marian doctrines and feasts, the simple fact remains that above any of the speculation (and this is what we celebrate): a young woman had courage and openness to God's will. Certainly there have been many before her, like Abram, who trusted God more than they trusted their own heart and mind. Each brought a new Covenant, enabled God's will to enter more and deeper into the World broken by the sin of Adam and Eve, but it is Mary who enables the ultimate immanence and Covenant: the Incarnation.
This moment is the moment that places her at the top of the Apostles; this is the moment that gives the ultimate revelation of God's will to the world; this is the moment that opens up the world to the grace that begins to save the world so damaged by sin.
For each of us, it celebrates the moments that we too are able to open ourselves up to the the will of the Father. We are Mary, allowing God into our lives knowing only the outcome of God's saving power but none of the details, joy, and heartache to come. We are Joseph, given the news of this mystery yet acting upon it with righteousness aware only of the necessity for action.
Every time we imitate the Holy Family, we acknowledge the gift of grace from God and the gift of Mary's "yes" which now allows us in our Baptism to also say "yes" to the Father in our own lives.
Do nothing out of selfishness or out of vainglory; rather, humbly regard others as more important than yourselves, each looking out not for his own interests, but [also] everyone for those of others. Have among yourselves the same attitude that is also yours in Christ Jesus, Who, though he was in the form of God, did not regard equality with God something to be grasped. Rather, he emptied himself, taking the form of a slave, coming in human likeness; and found human in appearance, he humbled himself, becoming obedient to death, even death on a cross. Because of this, God greatly exalted him and bestowed on him the name that is above every name, that at the name of Jesus every knee should bend, of those in heaven and on earth and under the earth, and every tongue confess that Jesus Christ is Lord, to the glory of God the Father. So then, my beloved, obedient as you have always been, not only when I am present but all the more now when I am absent, work out your salvation with fear and trembling. For God is the one who, for his good purpose, works in you both to desire and to work. Do everything without grumbling or questioning...
-- Philippians 2:3-14