(The following isn't just a mild rant, but an example of all the pitfalls involved in UPS-related purchases that don't initiate on a Monday or Tuesday!)
... So Let's say that on Wednesday you negotiate with a company to buy something. It will usually take them until the next day to drop it off to UPS or for UPS to grab it (Thursday) and enter it into their system (This is an East Coast to West Coast example; Roanoke, VA where this particular distributor (even though the purchase was made from Utah) has an outlet, which you have to believe is near a flight to take it out of there..... and heading into Silicon Valley (San Jose Area)). So you now have a record that at 9 PM on Thursday, UPS sends you out a message that the delivery phase has started and grabbing of the package has ended.
So now you wait, getting a number of messages, saying the package finally made it to Sacramento (I presume the end of the "flying" phase because it's the second night of flying, but maybe not and one more hop to good, ole San Jose!). Nonetheless, your package arrives at 7 AM on Saturday for San Jose processing (at a location that isn't open on Saturdays, so forget about redirecting or picking it up, for all practical purposes) ... By the way, Saturday was considered a "Business Day" when I searched other people's experience, but it may not be, anymore ... Also note that apparently there is a check box when the seller orders the Second Day Service, to allow for Saturday Delivery, so refuse to make the Wednesday purchase with the buyer unless they check the box (if you really need the residential item/package)! They may get charged more for allowing it, but you saved them the embarrassment that they lied to you when your package will really arrive!
Let's summarize ... UPS processes your package on 3 business days, T-F-S (if indeed Saturday is still considered a business day to them) and your package is going to sit in their destination facility starting from 7AM Saturday, to be delivered sometime (possibly) Monday Afternoon, or 5 days. Why does it sit there on Saturday when UPS delivers to residential customers on Saturday? Most likely, the UPS Truck Delivery Person loaded up their truck at <=6 AM, to make their Saturday Deliveries!
How do I know all this to be roughly true? ... I'm here without my package on Saturday (about 2:30 PM local time) and there is no indication that the package is "out for delivery" and the estimate is still Monday Morning/Afternoon. I even caught the person that does the local route in my area on Friday, and asked them about 5 day delivery of second day air service (they handle the route Saturday, too!), with a dumfounded facial expression. I even told them I would like to see the 1lb package on Saturday if possible, knowing this would be the fate/outcome.
... If it really arrives Saturday, I will modify/append this post. Did I end up really needing the package on Saturday? No, but I wasn't sure of that until USPS didn't deliver something I really needed more!
... I hope this gives a number of you an idea when you'll receive something UPS Second Day Air! ;)