Monday is the beginning of the workweek and school week and thus can be people's least favorite day. This was described specifically as "Manic Monday" in The Bangles 1986 hit song. As such, people may refer to Monday as "Manic Monday." A person could say on a Monday: "Today is not going well; I guess it's a Manic Monday."
Listen to the song with lyrics: https://www.youtube.com/watch?v=NVGOyYyWxiA
