I would suspect there are a number of factors that will affect whether the grass survives.

I think one of the biggest factors would be the intensity of the sun on the floor. If the floor is in direct sunlight on a hot day, I think the grass would be cooked.

I know it takes a several days to kill grass by placing a plastic tarp over it. but again it depends on the intensity of the sun.

So I think the decision is going to be yours and how much you are willing to risk to have a couple of hours of dancing at your party.

Where do you live?
Is the dance floor in full sun or shade?

If it does kill the grass, the only solution will be to replace the sod. Dead fescue will not return and the stuff at the edges will not spread to fill it in.

You say its "new" lawn....well....I guess the question is..
HOW NEW?.....
If you place a wooden floor....let's assume a size...25 by 25....and place it on top of new grass which denies sunlight and moisture from gaining entry, then you have to assume some damage might result.
If you can put down the floor....within a reasonable time before actually using it, then the lawn has a better chance of bearing up to it.....as long as....as soon as the hijinks are done with....the floor is picked up and fan raked to get the grass sitting up properly to allow sunlight and moisture to gain entry, no damage might be the outcome.

"I think one of the biggest factors would be the intensity of the sun on the floor. If the floor is in direct sunlight on a hot day, I think the grass would be cooked."

I don't understand this at all. The floor will block all sunlight, so intensity will make no difference.

One of the ways a lawn is said to be able to be 'cooked' is by way of a black plastic sheet put in such a way that the sun is directed to bear down on it.
It takes time.....even this wont kill a lawn in half a day.
I cant see a wood floor doing so terrible damage in such a short time to kill a lawn from that recovery cant be expected when the sun returns..