To the original question, I would guess the others are spot on with the tracking info provided lagging behind the actual movement of the package. In addition, it's likely the package sat in the office where it was dropped till end of day on the Saturday when it was dropped off, then shipped to a sorting facility where it remained until Monday morning. I do not believe USPS works on Sundays.
Not directly related, but of note, the USPS is one of only a hand full of government services actually allowed, and directly provided for, in the Constitution, yet since 1971 the USPS receives no tax money and must sustain it's self only on the money it collects for services. Perhaps, as a people, we should ask our congressmen to stop funding things that are not allowed, or forcing people to buy products they do not want to purchase, or bailing out private businesses, and instead cut the USPS a check for $1 trillion, and see how service improves.