If I'm trying to build something, I generally stick with it until I've got it working, so that can mean Sundays and many many Sundays, and many holidays, etc. I know, that's not healthy, moderation, blah blah blah.
I'm currently Fiat mining again so I'm trying to make the most of that. And for a little while my rule was to not work Sunday, and instead sleep late, veg for a little bit and maybe do some shopping or dinner with family or something like that.
However now I'm so mentally wiped by Friday evening that Saturday is the day I sleep in, veg a good chunk of it, and then Sunday I'm right back doing work a half day or more, cuz I'm refreshed.
The exception is when it's summer temperatures If it's nice weather on both weekend days I'll take Saturday and Sunday off, do some bicycle riding, or some other reason to be outside. Where for a weekend when both days are bad, then just the opposite -- I will work both days quite often.
And then if only one day is going to be good well then that's the day I take off and the other day I'm at work, whether it's Sunday or Saturday.