I want to return an "X" in column Y if column 0 has the number 21 thru 27 all
other values return nothing.

column o is currently a caculated field with the formula:
=SUM(TODAY()-P5)

column O is for number of days and invoice is past due
column P currently has a date field.

I want to be able to identify accounts once a week that hit the 21-27 day
past due days so that I can write a letter to these customers.

What formula would be best to do this?