You will be fine for a few days with tap water. But when you add the 50/50 mix of antifreeze and water make darn sure you use the best water you can put in your radiator which is either totally pure deionized water, or truly softened water. If you home tap water is soft water then that is what you should use. If it is hard water then get some soft water from someplace else. Both of these waters are devoid of calcium and magnesium. These are the hardness elements and this is what ruins radiators. and not tap water. Do not use distilled water in a radiator. The problem is that when water is distilled, or “stripped,” of its minerals and impurities, the resulting solution is composed of chemically imbalanced “ions.” This leaves distilled water “ionically hungry,” so it will actually strip electrons from the metals in a cooling system as it attempts to chemically re-balance itself. As it chemically removes electrons from the metals of cooling system components, distilled water eventually does extreme damage that could lead to cooling system failure. Also make sure you use the correct antifreeze for your make vehicle.