We say "a Monday" (or whatever day of the week) when we want to say that the event in question will occur on an unspecified Monday.
For example, if you said, "The next meeting of the Zetetic Society will be on Monday", that would normally be understood to mean the next, upcoming Monday. If today is Friday, that would mean three days from now. But if you said, "The next meeting will on a Monday" -- with the "a" -- that means that it will occur on Monday, but not necessarily the next Monday or any other particular Monday. Just that the day it happens will be a Monday.
Point of clarification: "The meeting will be on Monday" USUALLY means next Monday, but it could mean some other Monday if the context makes that clear. Like, "The preliminary meeting will be on June 15. Let's see, that will be a Friday. So then the full meeting will be on Monday." Presumably here "Monday" means "the Monday following Friday, June 15."
Best Answer
An adjective can often be used with a determiner (in this case the definite article) to become a noun that refers to all things that can be described by the adjective.
For example:
or:
or even:
A more sensible use of "the spiritual" might be:
Here "the spiritual" refers to "any or all spiritual things".
In the video, the joke is that "the spiritual" is taken to mean sweaty body odour. I shows that she doesn't understand or believe yoga's spiritual value and sees it only as a sweaty exercise.