For as long as I can remember, I've had older generations telling me that if you salt your meat too long before you cook it, your meat will be tough. Where does this stuff come from?? I'm sure somewhere, someone had a bad experience when they salted their meat, but nowadays we know better. Salting your meat actually works as a light dry-brine and can make your meat more tender. Not less.
Now when I say meat I mean beef, pork, chicken, lamb, etc. NOT fish or seafood. Just to make that clear.
After you buy your meat, it would behoove you to get home, unwrap it, salt it liberally with KOSHER salt, and then re-wrap it and toss it back in the fridge until you're ready to use it. But remember, you want to take your meat out of the fridge at least 30 minutes before you're going to use it. Some people may scream 'food safety!', but trust me, you'll get a much more even cook on your meat if you let it come almost to room temperature, AND if you're cooking your meat properly any food safety woes should be taken care of.
If you're able to salt your meat 1 to 2 days ahead of time, that's great! But I know not all of us are planners ;) So if you only have the 30 minutes before you're throwing that steak on the grill, that's fine. Use it. You can thank me later :)