Yes! You can make a sheet cake about 24-48 hours before you need them. You'll just want to store them in an airtight container or wrap with plastic wrap so they don't dry out. If you want to bake the sheet cakes even further in advance, say about 2 weeks ahead of time, you may freeze them for about 3 months without noticeable changes in flavor. Just remember to let it thaw overnight, then decorate as desired.