The way I'd probably go about it is to work out the date for the most recent Saturday, then subtract 91 days off that. I haven't tested it, but it would be something along the lines of:
[code]
[YourDateField] between
_add_days( _add_days(current_date...
[code]
[YourDateField] between
_add_days( _add_days(current_date...