It's Monday can have many connotations which vary from positive to negative. The most common is when having a weird day just to throw out an "Its Monday" to validate the weirdness or shitness of the situation. Unfortunately "It's Monday" cannot actually be used on a Monday as thats just wrong .
"Mate today has been the weirdest bloody day"
"It's Monday"

"This is the biggest Monday job"
by kingbuck October 6, 2018
Get the It's Monday mug.
A day generally created for the purpose of making people wish they were someone else. The day you realize you have 4 days of work ahead of you and that they won't be going by fast at all. Symptoms generally include feeling like crap, wishing you were dead, or not showing up for work in general.
"Sounds like somebody's got a case of The Mondays." ~ Office Space
by Kmetzman April 5, 2006
Get the The Mondays mug.
The day kids dread going back to the hell hole called school
Tomorrows Monday!
Nooooo not the hell hole!}
by Levi........ January 20, 2019
Get the Monday mug.
-An excruciatingly long unit of time
-Another way to say never.

This originated by a Playboi Carti tweet, only saying: “.MoNDaY”. To most fans expectations, nothing has happened on Monday.
Joe: yo man, my ex asked me when we were getting together, I said: ‘.MoNDaY
Tony: Damn, That’s Cold!
by efy2 April 24, 2020
Get the .MoNDaY mug.
The worst fucking day of the week. It should go crawl in a corner and kill itself.
by The What Race Am I? Guy March 24, 2015
Get the Monday mug.
The biggest waste, of exactly one seventh of your life.
by Eric Gardner November 28, 2007
Get the monday mug.