500baht overstay fee and an overstay stamp in your passport. That’s the worse case scenario. If they are feeling generous, they will just give you the overstay stamp and not charge you.
you’ll be fine. If your stamp expires on the weekend, as long as you’re at the immigration office the next day they are open, you won’t have to pay for an over stay.
Even better yet, why not go during this coming week? You don’t have to wait until the last minute.