How long can I keep Ceviche?
Ceviche is one of those things that just does not keep well in the fridge (and whatever you do, RESIST the temptation to freeze your ceviche!).
if you cannot finish your ceviche within a a day or two of making it, I would suggest simmering the leftovers in some tomato juice (or V8) or fish stock, with some white wine, thyme, garlic, etc., and making a "faux" seafood bouillabaisse.
moreso a problem than being at room temp, the acid in the citrus continues to "cook" the fish. it will be mushy the next day.
if you had the party yesterday, you might consider draining the fish, (like immediately) then drying it well with paper towels, dipping it in flour and frying it tonight for fish tacos. or else chuck it.