Yeah, the "weekend-ness" of Friday is interesting. Actually I could extend that to the last couple days, in relative time. For example, if it's Thursday, and I said "last Monday", that would actually be kind of confusing, just as if it were Sunday and I said "last Friday". I'm much more likely to say "On Monday," or "On Friday," if it's within the current week or the current weekend, and use the sentence context to figure out if I meant the previous weekday or the upcoming one.

I'm glad I don't write either dictionaries or voice-based/natural language apps for calendar appointments. ;-)