In Which We Apostrophize about Apostrophes

I've noticed a curious, not to say disturbing, trend in the recent online behavior of several correspondents.  When discussing activities conducted on a regular, weekly basis, many people now include apostrophes where, as far as I can tell, none are required.  For example, someone writes, "On Monday's I take my sloths to the sloth-washing facility for their weekly sloth-wash."  There's no reason for that apostrophe 's': The writer is not saying "On Monday is I take. . .," nor is she describing something in the possession of Monday.  The right word is "Mondays."

(I note, by the way, that when I typed "Mondays" above, I was not even chastised by Microsoft's famous red squiggle, further proof that the pluralization is perfectly acceptable.  Although, "pluralization" is apparently not, but that's another story.)

I hasten to point out that the people engaging in this random apostrophizing are generally well-educated and, indeed, skillful writers.  Whence, therefore, this unnecessary punctuation?  Answers are welcome.  Write me any time.  I can be reached on Mondays, Tuesdays, Wednesdays. . . .