You know what is really wild and caused a few bugs in corporate reporting apps? Every 5 or 6 years you need to account for a 14 week quarter. 13 * 7 is 364 so four 13 week quarters will not add up to a calendar year.
6-week December threw off a bunch of reporting last year for my company. The reporting group was not aware of the 6-week fiscal month, and the date dimension table didn't reflect it.
Thankfully no particular harm was caused by it.