Some performers recommend picturing your audience naked. That would tend to distract you from your stress. Or it could lead to pukeing if you have the kind of audience I'm used to playing for. For me, the love of playing music is always stronger than my fear of the crowd.
For some, a drink before going on helps, but I don't advise that. You will need all of your higher brain functions intact to properly control your theremin. I'd also avoid too much coffee.
For some, myself included, a jam session before the gig can steady your nerves quite a bit and get you "in the grove". If you have time during the sound check, play a song or two with the other performers. The goal is to become comfortable with your surroundings and band mates.
Make sure you have everything you might need well ahead of time. I'd rather bring things I don't need than be short of anything. When in doubt, throw it in the gig bag. Give your theremins plenty of time to warm up, and yourself plenty of time to relax and chat with your fellow musicians before the first show. In other words, remove as many stress inducing factors as is practical.
Have confidence, grasshopper. You are good. Very good. You have a much larger audience on You tube and Sound cloud. You just cant see them. Try to turn your fear into excitement. Try to go from "OMG, I have to play in front of people" to "WOW, I GET TO PLAY TONIGHT!"
You WILL make a mistake or two. That's part of live music. I've never played a perfect set, and I never will. Just accept that and move on, or even better, grin and pretend you did it on purpose :)
Also remember you have a huge support group right here and we all know you will do just fine. We are all sending prayers, happy thoughts, good Karma, and whatever else is appropriate.