Not a teacher
The word itself can be mixup or mix-up.
1) I got the days mixed up and (I) went (for) to my doctor's appointment on the wrong day.
2) I must have mixed up these two days, and went (gone) there on the wrong day.
3) I always mix up (not mixup) Saturday and Sunday because I am off work.
4) I always confuse Saturday with Sunday because I am off work.
5) There was a mixup, and I went to my doctor's appointment on the wrong day.
I is always capitalized. Days of the week are capitalized.
Off work is more common usage than off from work, I believe. *
Student or Learner