The obvious ones count: medical exigencies, family emergencies, and traveling with a college team to a competition. Be prepared to prove that these conditions exist. I have been known to accept other excuses, but only rarely. I can't write an infallible rule in advance, so please contact me (or your section leader, if you have one) as soon as is practical.
If a situation like this arises for the final paper of the semester, during exam period, you will need to take an incomplete. Please contact your dean's office, explain the situation, and apply for an incomplete. I will sign off on any excuse your dean's office will accept.