Your normal day is a weekday. Those represent the bulk of your life.
Weekends are not representative. They should be identified instead as occasional time.
On the case of side-projects, you shouldn’t rely on weekends, vacations or off-days to do the work necessary.
This kind of days is there for a reason. To be an opportunity for rest, to cast no pressure upon your mind, to be available to be filled with whatever arises on that very day.
You may allocate time to work on your side-project on these days, but you should only do that on the said day, if the mind and heart are willing. Otherwise, it is a defeated purpose of the rest days.
Instead, do your work on the weekdays. You will be more up to it, having momentum.
And it will be an extension to a pre-existing condition within the unit of a day, instead of an extension that surpasses the day. That tends to get to feel like a burden. And you don’t want that.