Hi,
“If yesterday was tomorrow, today will be Friday.”
–What day do you think was implied?
Work backwards…
If today is Friday, tomorrow is Saturday.
If it was Saturday yesterday, then it is Sunday today.
Some are still living in Wednesday. I wonder if it was any logic in counting that way. (My brain goes into overdrive deciphering it). 
I suspect that’s the whole point. 
You mean it’s an exaggeration referring to it as a brain?
Not at all! As if I’d say such a thing about you.
I mean the puzzle is intended to fry people’s brtains when they try to work it out.
Then may I beg for making it a habit asking you what day we’re really having? (Just to prevent what remains from overfrying). 
As I indicated in message #2, I think the required answer is ‘today is Sunday’.