What are the calendar dates of the first and twelfth days of
Christmas? 

I want an answer that's authoritatively sourced, and not just announced. 

My web research finds three groups of answers: (a) Dec. 25 & Jan. 5;
(b) Dec. 26 and Jan. 6; (c) weasely formulations like the twelve days
"from" Christmas
"until" Epiphany. 

I amazes me that there can be so many web accounts of the origin and
meaning of the twelve days (secret catechism, etc.) without pinning
them down to actual calendar dates.

The few that pin the dates down don't make clear whether the precise
dates are the current author's guess or the words of whatever source
the writer is using.

I fully realize that the date of Christmas is a big topic, with Dec.
25, Jan. 5, and Jan. 6 all being dates of historical observance at
different times in different places.

One explanation of the twelve days said that "Christmastide" was an
invention to bridge the gap between Christians who kept different
dates.

That's all very well, but the point is that there are only eleven days
*between* Dec. 25 and Jan. 6, so to reach the magic number twelve, you
have to include one terminal day or the other, but not both.

I recognize that some groups of Christians may celebrate a different
but  overlapping series of twelve days. If that's the answer, I'd like
to know the source of the information.

A related issue, or perhaps the same issue, is "when is Twelfth Night?" 

The Merriam-Webster online Unabridged equivocates maddeningly: "1 :
the eve preceding Epiphany marking the end of medieval Christmas
festivities
2 : the evening of Epiphany." 

Oh, so it's (a) the evening of January 5th and (b) the evening of
January 6th? How helpful.

I even wondered if it had something to do with the difference between
counting days from sundown and counting from midnight, which is the
key to how the Easter Triduum (three days) can consist of Maundy
Thursday, Good Friday, Holy Saturday, and Easter Sunday: count from
sundown on Thursday till sundown on Sunday. Voila! Three days.

But nothing I found on the twelve days of Christmas hinted at such a
solution. It seemed a frail hope, because any culture that has
"Christmas eve" distinct from Christmas is unlikely to count days from
sundown. (Cf. "Hallowe'en" and "All Hallows Day.")

I also wondered if the answer had something to do with the uneven
adoption of the Gregorian reform of the Julian calendar by various
countries, Catholic, Protestant, Orthodox, but no breath of that
emerged.

For clarity's sake, let me say that I am totally uninterested in the
meaning or history of the song about the pipers piping, etc. I just
want to know the calendar dates of the twelve days of Christmas, and
who says so.