First I’m hearing about it.
I saw your pic somewhere and did a double take. Isaac is white, and I pictured him with Einstein hair. Not close.
To the extent that this is true, I’d have to go with it being one of those “the skin on your elbow is your wenis” kind of word-usage “facts” that eventually become descriptivistically true through popular repetition, but have no basis in actual etymology.
Of course, while the wenis thing is harmless (if stupid, and increasingly annoying every time one of your oh-so-linguistically-gifted acquaintances trots it out), this “Mondays” business smacks of a trollish movement to cause everyone in America except drewtwo and apparently six other people to accidentally brand themselves as racists.
OP, your original insticts were correct: smile, nod, get on with your life, and keep hating Mondays with the rest of us.
Isaac and I shared a love of sci-fi and a fear of flying. It is not for me to judge whether you’d have a hard time telling us apart on the street. 
Well, you both seem like cool people I would like to party with. Any day of the week.
Damn, that’s weird, and no, I’d never heard it.
I wonder if it started as an internet bullshit meme (a la ‘picnic’ is a derogatory term), but some white idiots started actually using it. I can’t imagine how else it came to be.
I have hear ‘Martians’ but referring to Jews. Well, I’d never heard ‘Martians,’ actually, but my Jewish neighbor clued me in.
White people shouldn’t be the last to know about their own coded epithets!
This thread reminded me that in my childhood we had a black cat named “Monday.” I swear to Og I am not making this up.
Tuesday Addams was white. Just sayin.
So was her sister, Wednesday
As a card-carrying White Male of European Descent, I have to admit…that this is the first time I’ve ever heard of this. Of course, I am Catholic, so maybe that’s why I didn’t get the memo.
I’ve never heard of this Monday code.
Tuesday is Toes Day.