Another way of saying "night before"?

You know how when we read novels or historical novels of sophisticated writing, some people called the "night before" a different, fancier word? such as, "it's the ___ of the ball." I'm trying to say the night before the ball but I keep forgetting what the word means. I tried "fortnight" but I believe fortnight doesn't mean the night right before. Or how about another way of saying the day before? Help you Old English experts?