Я решил это.
Month1stSunday = 8-Date.DayOfWeek(Date.StartOfMonth([FieldDate]), Day.Sunday)
if (Date.Day([FieldDate]) <= [Month1stSunday]) then "W1"
else if (Date.Day([FieldDate]) <= [Month1stSunday]+7) then "W2"
else if (Date.Day([FieldDate]) <= [Month1stSunday]+14) then "W3"
else if (Date.Day([FieldDate]) <= [Month1stSunday]+21) then "W4"
else if (Date.Day([FieldDate]) > [Month1stSunday]+21) then "W5"
else ""
Может быть не очень хорошо, но отлично работает.