FedEx is not open on Sunday so you would not be able to drop off packages on Sunday to FedEx.
You can ship FedEx on Saturday and it will then travel through their sorting facilities on Sunday but the regular shipping FedEx offices are not open on Sunday.
So if you want a package to ship on Sunday you need to ship it on Saturday.
FedEx and UPS as well as USPS packages travel on Sunday even though the main offices are closed on Sunday but you cannot drop off packages to USPS, UPS or FedEx on Sunday.
But they will still travel on the plane, on trucks etc to reach it's destination.