To me, it looks like the work schedule for projects defaults to weekdays, and there is no way to adjust the default.
For companies that are on a different schedule, it would be really handy to be able to set this for the entire instance. Because that’s not something you’d want to have to set for every single project.
(If I’ve overlooked this feature, I’d appreciate someone pointing me towards it.)
Related: Enable setting a work schedule in project templates