Ow la la!

Fancy dinner parties.  The people you have to be courteous to.  The fancy-schmancy courses of food, the pristine white napkins and gleaming china and silverware... and you're on your period and cramping like mad.  All you probably want to do is scream and cry, but that wouldn't go over well in a situation like this.  (Don't try this.  It wouldn't.)
So you can't scream and cry.  Curling up and whimpering is also out.  So is complaining to other people who get periods.  What do you do?
As always, the first thing is that you should have been prepared in the first place.  If you're allowed to take painkillers, take one before you go, or bring a couple with you.  If you can't, use a heatbag.  (See the post on this). 
What if you tried all this and it didn't work? You need to excuse yourself.  Fancy dinner parties will nearly always have a bathroom, so go.  Whimper all you want- and cry and curl up if you have to.  (But don't scream, they could hear you).  After you've done that, if you have a tampon in, and if it's possible given your specific situation, take.  It.  Out.  I promise you that this will help.  Cramps are caused by the muscles of your uterus working extremely hard, and if they're also working to hold in a tampon, this puts undue added work on your muscles.  (And causes more pain!)  Failing that, try to go to the bathroom.  I don't know why, but if I can, um, empty myself out a little it helps.  Maybe it's because I'm not as full and so my muscles have more room to work in?  Like I said, I don't know.  After that, stretch.  When you're exercising and you get a muscle cramp, what do you try to do to get rid of it?  Probably stretch, right?  This works the same way.  It'll hurt like crazy, but you should arch your lower back and bend your stomach back as far as you can, focusing on pushing your hips outward.  (Put your hands on the ground for support if you need to).
This might not get rid of your cramps completely, but it should at least help make them (and the fancy dinner party) a lot more bearable.