First off - Welcome!

You need to experiment with the flash BEFORE the event. Your question suggests that you are very new to photography and your new equipment

What EXACT equipment will you be using?

Here's some general stuff:

Try using the flash on automatic in a similar setting...then see the results.

Then try using the flash at minus 1 or minus 2 stops and see the results.

You need to become familar with your flash and camera and the ONLY way to do this is through practice and reading the manuals that came with your particular gear.

You need to play a little bit first...DO NOT wait till the day before.

If you want more/better advice post a few test pics here...or post more precise questions.

Good luck
Marko