Aristotle A. Esguerra, the "Recovering Choir Director ", posted this in 2005.
The money quote:
No, it’s not a Holy Day of Obligation. However, consider that without the Annunciation, there would be no Christmas. It’s not as simple as that, but it is as simple as that. The obligation is on your hearts, not on paper. Do with it as you wish.Yes, God could have chosen another way for His Son to come into this fallen world. But He willed for us to be saved by Someone like us in all things except sin. To paraphrase St. Augustine, while we were created without our permission, we cannot be saved but with our permission.
So Mary's "yes" to God is our example, the penultimate expression of "Thy will be done," the first instance of her leading us to Him.
We would do very well to emulate her.