Simply because the weekend is over and it’s time to go back to the “old grind” (whatever that may be for you).
Today was excellent however. Yesterday was gloomy and rainy and just a bad day in general for me. Today was sunny and warmer and since I don’t have class until 1, I got to sleep in. :D I also have my two favorite classes on Monday. I love Monday this quarter.
One reason, I think, is because our—at least my—sleeping pattern changes over the weekend. This makes Monday harder physically for me than other days of the week because usually my sleep was less restful the Sunday night before.
I think it is probably because you don’t like your week day job. Perhaps if you had a job that was from Saturday through Wednesday, you could hate Saturday instead.