If the package is sent overnight and the shipper of the package has specified overnight delivery then yes the UPS will deliver the overnight package on Saturday.
I've had UPS overnight packages delivered on Saturday.
Normal deliveries that are not paid for Saturday delivery will only be delivered on Monday through Friday and not Saturday.
But when a shipper pays for and specifies Saturday overnight delivery which is more expensive the packages will be delivered on Saturday to your address.
All other deliveries will take place on Monday through Friday which is the UPS normal business hours.