I am to get my period tomorrow bit had unprotected sex yesterday. should i buy plan b?

Probably not: If your periods are regular- you should not have to worry about conceiving with your period due in one day. You have 72 hours after unprotected sex to take the Plan B - may be reasonable to wait and see - if your period starts normally you don't need the medication. On the other hand, if you don't want to take any chances, if you take the Plan B and don't need it, it will not do any harm.

Yes, to be safe: Periods and ovulation can be irregular, so you never know. Plan b is probably the safest course.