On usps.com, I can find a particular post office and get its usual hours, but it doesn’t tell me whether it’s open or closed today (a holiday). Any suggestions?
Try calling.
The old-fashioned way? Call them! 
S^G
It’s a federal holiday. All post offices are closed, including the one you want to go to.
Go to the post office and try to open the door. If it’s locked, then the office is closed.
Alternatively, as others have suggested, try giving them a call. You should be able to find the number in the phone book or on the Internet.
Google postal holidays for 2008. They’re closed today.
This information is actually on USPS.com, though not any place obvious. From the Events Calendar page, there’s a link to postal holiday on the upper right.
Many post offices leave their door open even when there is no service, so people can come in and drop mail in the slot and check their paid post office boxes, or buy stamps from the vending machine. My local post office is such an animal, even though they removed the vending machines.