If it says it's been delivered, it may mean that whoever received the letter in the zip code you're seeing might have accidentally input it incorrectly. I used to work for USPS and we manually scan certified letters and would have to input it as received, etc.
Also, it might mean that your destination might be part of the hub that the "delivered" zip code it's in. If that's the case, should be there the same day or Monday if it's on Saturday. But, seeing as how it's a holiday on Monday, the letter will be delivered on Tuesday.