Making pork tenderloin day before party

Pork tenderloin takes maybe an hour to roast in the oven. Unless you are making a tenderloin with barbecue sauce (this is great cold!) I'd make it right before serving. I rub the tenderloin lightly with olive oil, sprinkle with garlic salt and roast it uncovered for an hour. Delicious and so easy.

I wouldn't, unless you can see serving it room temp - makes a delicious sandwich and I also like to do a riff on Vitello Tonnato, using roast pork loin - tenderloin would work too. But sounds like you want to serve it hot, so my thought is that pork tenderloin cooks so quickly anyway, I'd do sides you can make in advance instead. And you can marinate the meat and/or make a sauce for it ahead of time too. ChefJune is right about doing a braise/stew if you want a true do ahead - they actually improve with age.

If you follow your suggestion, the pork will almost certainly be overcooked and grey. I wouldn't cook it ahead. It doesn't take very long to cook in the first place. "Cooking ahead" is really intended for braises and stews.

I never have found pork tenderloin to be good on subsequent days as shortly after it has been cooked, perhaps due to the small size of the pieces and the relative leanness of the meat. I've cooked and eaten a lot of pork tenderloin over years, so this is my opinion based on personal experience.

Have I put reheated pork tenderloin on my plate and enjoyed it? Yes, I have. Is it something I'd serve to dinner guests? It depends.

Ultimately this is a judgment call on your part based on how you feel the quality decline is, how important you feel these party guests are, your ability to manage your dinner party menu/multitask, etc. I assume the pork tenderloin is the main course and thus the star of the meal, so I'd be hesitant to compromise with something that will be so heavily scrutinized.

Anyhow, I suggest you do a test and decide for yourself whether or not the resultant dish is worthy of your vision.

Personally, I'd cook the pork tenderloin to serve for the meal normally (right before serving) and look at the other dinner menu items to see if time can be conserved elsewhere.

But that's just me...