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;
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