If there is no sales on Sunday, filter Sunday's data out of the input, and use ID INTERVAL = WEEKDAY1W. The default seasonality for WEEKDAY1W is 6 (Monday to Saturday, the 1 in the interval stands for Sunday). If the data only consists of Monday to Friday's sales, use INTERVAL = WEEKDAY16W (1 for Sunday and 6 for Saturday).
Here is a sample code to use INTERVAL = WEEKDAY1W:
/*generate test data with sales from Monday to Saturday only*/
data test;
format date date7.;
starting_date = mdy(1,31,2016);
do i =0 to 700;
date = starting_date + i;
if mod(i,7) ne 0 then do;
sales = abs(100*rannor(12345));
output;
end;
end;
drop i starting_date;
run;
/*use WEEKDAY1W interval to handle 6 days a week data*/
proc hpf data = test seasonality = 6 out= _null_ outfor = outfor;
id date interval=weekday1w;
forecast sales;
run;