Bananas are actually ripe and ready to be eaten when you see the first black marks appearing on their skin. Before that, you can eat them but they will require extra energy to be digested. If you receive your bananas unripe/too green, just wrap them in a sheet of paper and put them in a cupboard for 24 hours. As the ethylene the bananas emit when ripening is trapped inside the paper, it will allow the ripening process to accelerate. Your bananas should be ready in a day or two.