Visage Day

With the rise of mortals like humans, elves, and dwarves, the dragon's ability to shapeshift allowed them to be within the mortal, and the Visage Day ceremony was officially created. A ceremony during which a dragon takes on his first humanoid form. This ceremony also generally shows the transition from adult to ancient dragon (by mortal standard).
This ceremony was first imagined by Alexstrasza after the war of the ancients, the mortal now able to roam the lands, and growing to be an important part of Toriel, Alexstrasza thought it would be a good idea for the dragon's to be able to choose a form to live among the mortal from time to time and to learn about those new people they'll have to share this realm with.