Tempered chocolate spheres in advance?

Chocolate will stay fresh for weeks or even months if stored correctly. And yes, the spheres will stay as shiny and beautiful as you made them if you store them correctly:

• in a cool, dark place away from sunlight
• but not necessarily in the fridge - taking them out will lead to condensation on the surface
• at rather constant temperature
• and dry.

So in short:
A large enough foodsafe plastic container in a cool cellar or cupboard, away from (sun-)light and, if applicable, curious kids, pets or others that might feast on your stash in advance. Unless you have a high humidity environment, it's not absolutely necessary that the box be airtight. But if you want to be really sure, include some silica gel packs against excess humidity.

Do not stack them like nested bowls, or you could scratch or otherwise damage the glossy surface. If you have to, place one layer in your box, cover with parchment, foil or similar, add a sheet of cardboard for stability and another layer of parchment or foil, then the next layer of spheres.

Good luck and have a good party!

I admire your ambition. I regularly make chocolates with filled spheres (piping in anything from caramel sauce and nut butters, to my own flavored ganaches). But I buy the shells already made from Chocolate Man (near Seattle). They use very good quality chocolate, and their prices are very reasonable (though shipping is a bit pricey, so plan ahead and order as much together as possible). This lets me focus my efforts on the fillings, and on dipping and decorating the results.

More directly responding to your question: I've used my Chocolate Man shells after storing for many months, and they've been fine. I've even used some that were stored for almost a year (stored in a box in a cooler part of my house, but at room temperature). Those shells showed a bit of bloom, but always taste one plain before using them after storage, and even those year-old ones have tasted totally fine. Since I dip the shells in tempered chocolate afterward, a bit of bloom is not a problem—the risen cocoa butter composing the bloom just gets absorbed by the tempered chocolate couverture. So I don't see how a few days or weeks of storage of your homemade shells could be a problem, as long as they are not stored in a warm location.

By the way, I'm referring to dark chocolate shells. If you use milk chocolate, you probably want to use the shells within a few months at the most.

Finally, I invested in a small wine cooler (one of the thermo-electrically cooled ones; they are very quiet and use little energy) so that I could safely store some of my better and more fragile bulk and prepared chocolates, esp. during the summer (I have no air conditioning). These coolers only go down to the mid 50s F, but that's plenty cool enough to keep even milk chocolate safe for months, without resorting to the lower temperatures of fridge or freezer storage (I tend to set it to around 60 F). So if you are really concerned, you might consider investing in one of these. If you do, plan ahead on how you want to store things in such a cooler. You may need to get containers or trays of appropriate size to fit in it.