I think a constant rotation would be too confusing to keep track of for people that aren't regulars on the server. If we want the highest possible participation/interest, then perhaps we could have one select weekend where the day is different, while the rest are standardized.
Ex: The third saturday of the month has the 'Classic Event Marathon', while all other weeks have it on friday.
That way we can make sure to let people participate at least once a month that are always busy on the standardized day (friday, in the example above.) If they're busy on both friday and saturday, then they need to get a better schedule
Edit: My suggestion above relates to a weekly marathon, but that is not what Thor was going for, my bad. A bi-weekly marathon should keep the day the same, in order to not force non-regulars to search for information.