Disclaimer: This is a work of fiction. I think. Probably.

Irene Taylor thought to herself "Monday, finally." On Friday, she had finished several days of prep work for her great spell. However, for some reason she didn't understand, the spell could only be cast on a Monday. She paused to center herself, and to check that nothing was obviously wrong. She did not wonder whether she should cast the spell -- that was certain, but did spend a brief moment wondering why she had not cast the spell long ago. She had only decided to cast the spell a week ago and could not think why she had waited so long. She was sick of being a woman in a man's world. The spell would solve her problem: it would switch everybody's gender, and they would not realize that it happened.

She cast. The spell worked; everyone's gender switched. However, the spell was not perfect. While no one remembered being the opposite gender, most did feel that something just wasn't right.

Jennifer Smith woke up. She just couldn't stand being a woman any more, and realized that she should cast the gender changing spell. She wondered why she hadn't done so long ago, and was annoyed that she would have to wait until next Monday to cast it.

Next Monday....

You say that this obviously doesn't happen, because you don't remember it happening? That just shows how good the spell is! And this does explain why Mondays always feel the way they do.