My issue was I wanted to disable Friday and Saturday so they cannot be selected in the datepicker calendar.
First open jquery.ui.datepicker in an editor like notepad++, then;
At line 92 you must replace this code
//  = true if selectable, false if not,  = custom CSS class name(s) or '',
//  = cell title (optional), e.g. $.datepicker.noWeekends
for this code (for disable friday and saturday)
var show = true;
The reason I'm sharing is I spent two days trying various different snippets of code in both the form html file and the jquery.ui.datepicker file and the above is the only thing I found that actually worked.
I kinda figured it might save someone a bit of time if ever they need to do this