fresh littleneck clams

Exactly what Chef June said. As long as the shells close if they are slightly open they are of course alive. If any of the clams do not open wide when cooked toss them out. Sorry.

The fish monger (or seafood dept at grocery stores) are required to keep the tag which tells where the clams are from and when they were harvested. The fresher the clams the more flavorful they will be. I scrub the shells under running water just before I cook them. Enjoy!

They should be closed tightly. Clams live in the sand without water each time the tides drop. Once open they should be discarded immediately. This rule doesn't apply to all shellfish.

For more years than I can count, I've made clams oreganato on either Christmas, or Christmas Eve. Typically bigger clams, Cherrystones, but I can't see why Littlenecks would be different. Whenever it falls on Christmas, I buy them a day ahead, put them on ice - but not touching the ice (as described)- and they're fine. I haven't noticed a higher percentage of dead ones (shells ajar) either day. I do buy from a trusted local fish guy. I don't clean them until the 'day of.'

Don't put them in water. They came from the ocean -- which is salt water. Unless you have sea water, water's not a good idea. Put them in a bowl and loosely cover it. then set that bowl in a bowl of ice/water. it will keep them fresh. When you get ready to cook them, check to make sure they're alive. If the shell is slightly ajar, knock on it lightly (who's there?). If the clam is alive, the shell will close. Discard any that don't close.

I've kept clams in a cooler or the fridge for a few days without a problem. I'd skip the ice, as you don't want them in water. Just put them in a bowl with a loose top. Obviously, discard any that are open, even a little. They should be tightly closed.

I have to assume you are purchasing from a fishmonger and not a supermarket. In the latter case, God knows how long they have been out of the water already.If you hold them overnight you are inevitably going to have some dead soldiers. If they don't open and sing opera to you discard them.
Personally I try to use fresh clams within four hours of purchase. I was spoiled by the fact that I used to live within two miles of a fish market where they let me reach in the seawater tanks and pluck out the smallest, most tightly closed little guys I could grab with my bare hands. Eating a clam that died before cooking can ruin your week and sometimes your life. Also, ask where the clams came from. Mollusks are subject to algae toxins (and man made ones) that can be very serious.

Clams keep 4-5 days out of the ocean so as long as they look fresh and alive at the fishmonger a day or two in the fridge shouldn't be a problem. I would rinse them quickly. In a bowl put a few ice cubes in the bottom and then invert a plate that will just cover the ice but not let the clams sink into the water when the ice melts(or you could use a blue ice pack if you had one in the freezer) put the clams on top and then on top of the clams a damp kitchen towel. Then put them in the fridge.