Why Saturday? They should cut out a day in the middle of the week. This way, you'll have to wait up to two days longer for a given item. It's better to distribute that wait time.
Not only that, but Saturday is the only time I'm home when the mailman arrives, in case of the need to sign for a delivery. By eliminating any intersection between the mailman and I, it greatly complicates the delivery of signature-requiring mail.
This has more to do with staffing and hiring than with getting us our mail in an optimal way. Most people don't like to work on Saturday unless they're getting paid overtime.