"The town of Ascoli was under papal rule and in 1482 Pope Sixtus IV granted it a degree of self-government. [...] News of Ascoli's new status reached the town on the feast of the Annunciation on 25 March, which then became a special feast day when the town celebrated its liberty."
The crisp detail in this work makes it just pop. There is so much going on that the eye is constantly pulled away from the Annunciation scene, or more specifically, Mary. Her room tucked away in the corner is like a hidden treat to be discovered and rediscovered as you look at the painting.
Mary is calmly reading, so the scene would seem to occur in the instants before she addressed by Gabriel. It is interesting to think that we are seeing her right before her life changes forever.
(I love the National Gallery's website. I'm constantly discovering new treats. This page has audio of a historian talking about the food shown in the painting.)