well... that "random day" is the day Rus is founded though, which is pretty much the poster boy scenario for the pagans just as William fighting Harold and Harald serves for the main game of western-centric feudalism (ie. vanilla core)
of course, Basil's coronation is indeed another important date, but it's irrelevant in relation to the main focus of the DLC scenario, ie. the northern pagans.
Didn't know that. However, Jan 1st exactly is a bit odd, especially when dealing with the quasi-legendary nature of the recorded history of the region at the time.