I'm sure my reply is too late for the wedding, but here are my recommendations when trying to pull yourself together when all you want to do is climb under the covers and hibernate. 

Take whatever medications you can without damaging your ability to perform, take a shower (steamy for congestion, cooler for fever), drink lots of fluids (I'm fond hot tea with lemon and honey soothing), and nap when you can (if possible).You must eat, but eat smaller, lighter meals more frequently. 

Most importantly, I hope you feel better soon.

Yessss, it works again to reply on posts!!!!

Looks like you got a lot of good advice:) I think, the main rules to improve the state is the following:

a) the hygiene-- shower, tooth brushing, you must feel fresh.

b) the appearance: as a girl, i use makeup. it will actually defocus one for one hour from that one is ill, if one feels goodlooking. so do whatever needs to be done to feel goodlooking

c) aspirin no headaches or shivering should be present while performing.

d) menthos! you must take care of the sore throat. before the performance, have a menthos...but dont mix it by having a softdrink nearby...you know what happens when mixing menthos and soft drinks?

http://en.wikipedia.org/wiki/S.....y_eruption

I wanted to ask: is there possibility for you to do an agreement with two friend violinists, that if any of you get ill you offer the work to each other? for such a thing as this wedding, it would be helpful sometimes to have the backup option to stay at home, but send there a friend instead of you...and he could do the same if he would get ill.