Best to have your new water mixed and heated before adding it to the tank with rock/sand. Add some starter bacteria as well such as Fritx, Aquavitro Seed or BioSpira.
Depending on how long it sits in freshwater some or all of the bacteria in the sand will die off. If you bought live sand that came in the bags then keep it in the bags until you're ready. If this is live sand from a tank taken down then I suggest just keep it in buckets, rinse it well and let it completely dry before you're ready to use. No point in trying to keep it "live" when you'll want to rinse it out for all the detritus anyways.