How far in advance can I shuck oysters for Oysters Rockefeller?

They should be shucked at the last possible moment. However, here's a hint for next year.
The oysters in the pint jars are just fine for Rockefeller. Get the around October when they're cheap by the pint and freeze them.
Save the oyster shells from this year. Boil them scrub them...take them outside and let the ants get to them...boil them scrub them..etc.
Then save them for use for baked oyster dishes like Rockefller. I have a bag of them I use every year.
You can also find little seashell dishes at Pier One and other places to use for oyster rockefeller and pre-shucked oyster; but the shells, saved and cleaned, from this year are much better looking.

If you're eating oysters raw, you don't want to hold them more than an hour or so, even on ice. However, if you're serving them cooked (e.g., for Oysters Rockefeller) they're find kept refrigerated on ice as Le Bec Fin describes, for 24 hours. Same for clams - in fact, I just picked up some Littlenecks to bake (clams oreganata) for a first course tomorrow. The fish guy shucked and put them in a container with their liquid, along with their shells in a bag, for stuffing (I'll scrub them first.) I've done this many times and it always tastes fresh and very delicious.

p.s. bottom left REAR corner of your frig.

you are basically asking the shelflife of refrigerated shelled shellfish . if you put them in a plastic container and keep it on ice, i would feel comfortable with a full day. no need to keep them in their shells, if you don't have room, because you can just put them back in the shells , top them and bake and serve. the bottom left corner of your frig is the coldest spot and the best spot to store meats and sfd. And with all seafood, 'on ice' makes a huge difference in enabling their quality to shine.

Once shucked keep them on ice in the fridge. If you keep refreshing the ice, you can do this an hour in advance