Saturday is considered a shipping day for USPS, FedEx and UPS.
However if you order something and the business is closed on Saturday then your package may not ship out until the following Monday.
Amazon packages are shipped all the time even on Saturday but some business may not take or complete orders on Saturday.
In that case the package will be processed and shipped out the next Monday.
However USPS, FedEx and UPS do ship packages on Saturday.
So if you're waiting on a package through either of them then the package should arrive to you on Saturday if the package is out for delivery.