It changes, but usually Saturday.
When I was a kid any day that wasn’t a school day was a good day. Saturday was the best, because you could sleep late, and stay up late.
I loved Thursday in my late teens. It was a go out to the clubs to go dancing night. Saturday also.
During the majority of my career I worked a varied schedule so again any day off.
The last few years Saturday was probably me least favorite, but all days were good. I do like Saturday still because my husband has the day off. Saturday is the one day I have nothing in my calendar to do. Funny, when I worked full time I relished a day I had nothing to do, and when I barely have a job, I relish the days I had something to do, but it was all fun things.