Is rain harmful to concrete slabs?

You’re asking that at below freezing temperatures at night? No concrete has been poured now, has it?

Guys, which covering foil can you recommend?

I would discuss that with your floor slab installer.
Do you have to apply it yourselves? Especially in cool temperatures, dealing with the foil is tricky. In summer, you "only" need it to prevent evaporation; and there it doesn’t really matter what kind it is; but if the temperatures are below 5 degrees, you have to use different methods. (e.g. insulating foils etc.).

You ask that at below zero temperatures at night? They haven't poured any concrete now, have they?

Yes.

Thank you very much for your answers.

You really have to know how to pour concrete in frost... Above all, it must have >10° during installation and be well insulated afterward. A foil alone certainly won’t be enough.

So with us it was like this, that the concrete plant does not deliver concrete when it gets too cold....

There are additives for all sorts of things, like retarders, accelerators (for frost), so as far as I know, concrete can easily be poured down to -5°...