Right, Halloween is a very pagan, secular holiday.

Just to think how messed up the world has become in the past 495 years!

I supposed Halloween could also be thought of as the day for contemplating (not celebrating) the dogma of Hell, which many deny. It's too bad Pius XII suppressed the Vigil Mass for the day because it would have been nice to have something to especially counter the ghoulish activity wrought during the time.

Pius XII suppressed the Vigil of the Feast of All Saints?  Do you know
what year that happened?  

I recall that my mother wanted to be a good Catholic, but she also
wanted to share with her children some of the fun she had as a child,
in her experience, and that included Halloween.  I have to give her
credit for trying to keep it 'innocent,' but I think she should have
seen the writing on the wall, as they say.  It is curious how the
occult has a way of creeping into your life when you just want to
have a 'good time' and some 'fun' with your friends. So she let us go
out "trick-or-treating," then she gently but firmly required us to keep
it all untouched until the next day, because the vigil of a holy day is
a day of penance and self-denial, or even fasting and abstinence (if
you carry it to the logical extreme), so eating candy doesn't really fit
in with that.  But keeping the candy for the Feast Day that follows
made sense to her.

Looking back, it could have just been a matter of economics, for she
was poor as a child, and it seems her mother may have done this, let
the kids get free candy and then they could have their fun and it
wouldn't 'cost' anything.  

Well, although it was a druidic celebration, the Church Christianised it. It is Hallow (Holy, i.e. Saints') e'en (even = eve). But as the society becomes more pagan, the eve of the great feast of All Saints becomes more wicked. In Australia, it was only ever a strange word and alien to us, but since we've had imported entertainment which advertised it, it's caught on over the years. The same for Italy. It didn't exist. It's real Americanism at its worst.

It seems the day as a secular quasi-holiday was not really in much practice at
the time of the revolt of Martin Luther.  There may have been a druidic tradition
already then, but there was no Halloween as we know it.  That came later, and
its start was in America, as you say.  Luther nailed his 95 rhetorical ramblings on
the Wittenburg church front door in 1517 long after the Church had already
established the Feast of All Saints.  

To say "the Church Christianized it" is to follow the false assertions of the
Protestants who look for every way they can find to be critical of the Church,
even if it means promoting lies.  Here is a pretty good reference for what history has
to teach us regarding what has really happened, instead of what heretics say
in their closed circuits of erroneous contumely.  

The Church, in fact, has been practicing All Saints' Day for over a thousand years.
And All Souls' Day for 500 years, started by the Dominicans.  Curiously, it was
just after that when Luther got his bright idea, and one has to wonder if his
choice of October 31st to vandalize a Church door had anything to do with his
contempt of the Masses being offered for the faithful departed.  But nobody was
trick-or-treating yet.  So the Church did not "Christianize" Halloween.  I know that
you didn't actually say that, but what you said could easily be interpreted to
say that if someone isn't paying close attention.