Even if your vet doesn't see patients on the weekend, he or she likely has a group of other vets who share weekend call? Or do you have a PetSmart in your area? They're open on weekends and have a vet on call, I think.

If yucky poopy = diarrhea, Immodium can be given to dogs (at least our vet gave us permission to when we called and asked) for short term relief.