Is there a word for the day before the last day of a period, which is measured by days?

Solution 1:

"Penultimate" means "second to last."

Solution 2:

In practice, though, I don't think I'd want to send out a notice saying, "This is your second to last chance to pay this bill." The recipient is likely to conclude, Okay, I'll wait until the last chance and pay it then. I think I'd say something more like, "You have now entered the final period in which you can pay this bill without a late charge" (or whatever horrible things you will do if they don't pay).

In a different sense, if there are two chances to win a game, I think I'd refer to them as "first" and "second" or "first" and "last", not "second to last" and "last". Saying that something is "second to last" rather implies to people that there are more before that. It reminds me of the joke of the runner who boasted that he came in second in the race while his hated rival came in next to last -- without mentioning that the two of them were the only ones in the race.