Oh, so it's about religion! It's nice, making a celebration out of that. There's a similar tradition in the kingdom of Hyland, back home! They pay respects to the Lady of the Lake, an important Seraph. [He grins -- the same Seraph happens to be the one he made the Shepherd's pact with, later on, too.]
The sanctuary was full of all kinds of decorations there, too. I guess it's just something that has to do with the spirit of celebration!