We recently had two two-day federal holidays, Dec 25-26 and Jan 1-2.
Naturally, the USPS did not deliver mail to our homes and businesses
during those periods. I assume that all letter carriers had those four
days off. What I'd like to know is this: Were *all* USPS employees off
on those days, and, if so, was all mail movement in suspended
animation at all USPS sorting facilities around the country? Was all
USPS sorting machinery simply shut down?

Even on major holidays, the USPS delivers stuff shipped by their
Express Mail service, so I'm sure that at least some USPS folks work
on those days to handle that.  Plus, in New York City the post
office's main branch remains open on holidays (I imagine other major
post office branches in other major cities similarly stay open on
holidays) so someone from the USPS is obviously staffing them.

Not that I have any official reference to point to, but I'm sure it's
not true that all mail processing and delivery simply stops when
there's a holiday.