I haven't had many inconsistent experiences with UPS, but USPS..
I like USPS. However.. Sometimes, they don't keep up with their tracking updates.
For example.. Let's say that they drop your mail off at 12 PM each morning, and your package arrived to them at 1 PM. Sometimes, they seem to mark it as 'delivered,' and deliver it the next day.
That said, I've watched the tracking of my package, and it hasn't been updated in about a week. Then it arrives.. A day later, all of the tracking information that was supposed to be filled in from those seven days, and the day it was delivered, was entered.
I think it's more likely that you'll get your package tomorrow. You can call the company that's shipping it, if you want, to see what information that they may have. If it's USPS, try calling your local post office.