Possible Clues:

• "This week has gone on long enough!" letters
• Letters for nine-to-fivers
• "Woohoo, the weekend!"
• "Finally, the weekend!"
• Late-week exclamation
• Four happy letters?
• Fri. mantra
• Thanks for Fri.
• End-of-week ''Whew!''
• Relieved end-of-the-week cry