Rolling average on index

Please try this expression.  It gets the correct result for Stores B and C on 2/17/2020.  However, since Store A had no customers on that day, it returns blank for Store A when the date = 2/17/2020.  To get around that, you'll need to add a Date table and make a relationship to your FootTraffic[Date] column.  Then you can adapt this measure to get the VALUES of your Date[Date] column instead.

Last 5 Open Days =
VAR __maxdate =
    MAX ( FootTraffic[Date] )
VAR __openlast5 =
    TOPN (
        5,
        CALCULATETABLE (
            VALUES ( FootTraffic[Date] ),
            FootTraffic[Open Day] = "Open",
            ALL ( FootTraffic[Date] ),
            FootTraffic[Date] >= __maxdate - 6,
            FootTraffic[Date] <= __maxdate
        ),
        FootTraffic[Date], DESC
    )
VAR __customercount =
    SUMX ( __openlast5, CALCULATE ( SUM ( FootTraffic[Customer] ) ) )
VAR __opendayswithcustomers =
    COUNTROWS (
        FILTER ( __openlast5, CALCULATE ( SUM ( FootTraffic[Customer] ) ) > 0 )
    )
RETURN
    DIVIDE ( __customercount, __opendayswithcustomers )

If this works for you, please mark it as the solution.  Kudos are appreciated too.  Please let me know if not.

Regards,

Pat