It can be done, but it is no simple task to calculate dates in fields.
However fellow MVP 'macropod' has done all the work for you and you can
download his work on date fields from the downloads page of my web site
which you can modify to use your {REF WeekEndingDate} field in place of the
DATE field and set the delay's required in place of the default 14.

Nor is it that difficult to do in vba either. The following, run on exit
from your date field WeekEndingDate, will get the content from
WeekEndingDate and subtract 6 and 5 days and paste the results in Textfields
Text1 and Text2 which should both have the property Fill-in enabled
unchecked

Sub GetContent()
Dim oFld As FormFields
Dim dWEDate As Date
Dim dDate1 As Date
Dim dDate2 As Date
'etc

Set oFld = ActiveDocument.FormFields
dWEDate = oFld("WeekEndingDate").Result
dDate1 = Format(dWEDate - 6, "dd/MM/yyyy")
dDate2 = Format(dWEDate - 5, "dd/MM/yyyy")
'etc
oFld("Text2").Result = dDate1
oFld("Text3").Result = dDate2
'etc
End Sub