Is this an iCloud email account?
You can login to iCloud.com then in Mail click the little cog at the bottom left and you get this screen where you can setup a "vacation message" that would do what you want. You would need to login to iCloud.com every Friday evening an manually turn this on then turn it off Monday AM.
-- Assuming working hours from 9 to 5
-- time of (current date) will return the number of seconds elapsed since 12:00:00 AM that morning.
------------Mail rule Script----------------------------------------------
using terms from application "Mail"
on perform mail action with messages theMessages
if weekday of (current date) is in {Saturday, Sunday} or time of (current date) < 32400 or time of (current date) > 61200 then
tell application "Mail"
repeat with theMessage in theMessages
set theSubject to subject of theMessage
set theContent to "Our opening hours are monday - friday from 9 to 5."
set theSender to sender of theMessage
set newMessage to make new outgoing message with properties {visible:false, subject:theSubject, content:theContent}
tell newMessage
make new to recipient at end of to recipients with properties {address:theSender}
end tell
send newMessage
end repeat
end tell
end if
end perform mail action with messages
end using terms from
------------------Mail rule Script Ends here------------------------------
(*
Next block is for running the script in Script Editor with some mail messages selected in the Mail applictation. If everything works remove the entire block and attach script to rule.
*)
-----------------ONLY NEEDED FOR TESTING FROM WITHIN SCRIPT EDITOR--------
tell application "Mail"
set myMessages to selection
tell me to perform mail action with messages myMessages
end tell
-----------------REMOVE WHEN TESTING IS DONE !!!!!!-----------------------