try this. I could not test as I have to wait for seven days to completely test. Today 14th October it works.
first type in A1
right click the tab of the sheet and click view code. and in that window copy this code
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address <> "$A$1" Then Exit Sub
If Weekday(Range("A1")) Mod 7 = 4 Then
Range("B1").Interior.ColorIndex = 3
Range("B1").Interior.ColorIndex = xlNone
everyday automatically the entry in A1 will change to that day
now when the week day of A1 becomes Wednesday the color in B1 will change to red.
check for a few days and confirm whether it is ok