The expected delivery date on USPS or other tracking information is the date that the package is expected to be delivered to your address.
Sometimes the package you're expecting can be delivered a day or so before the expected delivery date and sometimes the expected delivery date may change and it may arrive a day after the expected delivery date.
The expected delivery date is also just an estimate as to when the shipping company such as USPS, UPS, FedEx etc estimate that they will be able to deliver the package to you.
The expected delivery date is not a guarantee as to when the package will arrive but it's a good estimate.
I find that most expected delivery dates on my packages are pretty accurate and the package arrives on the date that it was expected too.