Only Individual timeshare facilities (i.e., not RCI or any other exchange company) can identify a "use week" for ANY resort.
In the U.S. (and, for the most part, in Mexico as well) 95+ % of all resorts are "Saturday to Saturday" weeks. The remainder are either "Friday to Friday" or, in very rare instances "Sunday to Sunday". In NO instance does anyone ever get to choose or identify their own 7 day week --- a "use week" is pre-defined by each individual facility.
KC