If you're sending packages out for the holidays, just don't try to mail them from a US Post Office on Wednesday, December 5th. Post Offices across the country will be closed and regular mail service suspended.
The reason? It's a National Day of Mourning following the death of 41 -- President George H.W. Bush. While limited package delivery will continue, not much else will be happening. And in fact, the National Day of Mourning is basically a legal holiday. So you can expect all federal offices to be closed on Wednesday, and most federal workers (except essential personnel) taking the day off. U.S. stock markets and even some banks will also be closed.
A National Day of Mourning is a rare event, typically done only when a President dies. And if it seems like it's been a while since that happened, it has been. President Gerald Ford died the day after Christmas, in 2006. And that's the last time the government shut down for a Day of Mourning.