If you have a break week then I’m assuming you’re on a combined pill. I believe that if you miss a combined pill, unprotected sex that happened prior to the missed pill is fine.
Even so, if this is your last day before your break, then just count this as an early break. Start your next pack a day early and you’ll be fine. Just make sure that you do not extend your break week longer than 7 days. So if you usually have a Monday start, start the Sunday before instead. You’ll be fine :)