(Funnily enough, if we're invoking ISO 8601, it also defines that weeks are anchored to a year by whatever year their Thursday is in.)
I used to go by Monday–Sunday, but I've grown into a firm believer of Sunday–Saturday. I'm going to start my own standards organization, and we'll have incredible tea, open access, and civilized boundaries for weeks. [relevant xkcd here]
Fair point that there's some ambiguity, albeit not caused by an inherent ambiguity between "this" and "next". I'd just invoke "next Thursday" in that situation because it's the same regardless of apostasy.