Since I'm a noob, is a bit risky to say what fits or not with MD reality, but this is my opinion on the matter:
From what I read in the announcements about the Days of Tranquillity, it seems this day is related with the night. I don't see connection with the cicle of night with a no sanctuary day, the day is already the currently day of MD, with the sanctuaries.
From my point of view, the use of an item/spell capable to break sanctuaries, would be much more interesting, and would fit better.
But well, well, what know I?