How to act at a friend's birthday meal when you are feeling tired and have had a really bad day at work with out bringing the mood of the party down (not going is out of the question)
Think of the birthday meal as a vacation away from your troubles. You'll be with people you like, in a festive atmosphere, not back at the office. I've been in the same boat but once I've been with my friends for ten minutes, I'm in the mood to party and enjoy myself.
It would be really mean to allow your bad day to cause others to suffer! Go out, let your hair down, enjoy the company of good friends and laugh. You can get an early night tomorrow! Nothing picks me up better than a good laugh - go for it.