I'm making meatballs and spaghetti for the first time and I plan to serve them on Sunday. Should I make them in advance or do they taste better if I make them and serve them the same day?
You can make meatballs in advance. You can cook them and then add to the sauce after you brown the meatballs. A day and a half is not a problem to keep the meatballs and sauce fresh. Actually, the flavors of the herbs and spices will develop nicely with a little time. The "better the next day" rule applies here!
can you store the meatballs in the sauce overnight or will it ruin the meatballs?
I would definitely store the meatballs overnight IN the sauce. That'll keep the meatballs from drying out and will impart a nice flavor to the sauce.
Agree w/ the above answer; however, make sure to use good, airtight, non-plastic containers for both the meatballs and the sauce to enhance flavor and freshness.
And re-heat slowly.
Honestly, I would make the meatballs ahead of time (i.e. make the meat mixture and shape into balls), but not cook them til the day you'll be serving them. You can make the sauce ahead of time--totally better the next day--but I would cook off the meatballs the day of. They don't take long to cook, and having them ready to go is more than half the battle.