The act of taking Monday off from work/class because you got crunk so hard over the weekend that you couldn't recover in time. The joke is that you're celebrating "St Monday's Day" and so you can't come to work.

The term actually dates back to pre-revolutionary America. Benjamin Franklin makes fun of his co-workers for taking a large number of "Saint Mondays" in "The Autobiography of Benjamin Franklin".

A buzz word being used to refer to creators and influencers forming or encouraging unhealthy relationships with their fans. Usually used in the context of young audiences feeling they have a relationship with the influencer due to the huge amount of the influencer’s content they consume. The influencer in question, however, is unaware of the fan’s existence other than the general knowledge of having fans. Common examples include Dream and his stans. These relationships are exacerbated by the influencers confirming they have more than a creator-to-viewer relationship.

The word is circulating due to various YouTube essays about the phenomenon using the word, and therefore people who enjoy complaining about influencers repeating what they heard in the video in any comments section they can find.