It's simple; you should always have an extra pack on hand anyway, so get your doctor to call your pharmacy so you can get an extra pack, and then just use one of those to fill in the extra day. If you're on a monophasic pill (only one kind of active pill plus placebos) you can take any active pill to fill in; if you're on something like Ortho-Tri, with the different hormone levels, you just have to make sure to take the correct one. It's not like you actually missed any pills, you just need an extra one for that 21st day of your pack.
Call your doctor! They deal with this stuff *all the time*.
Wendy