The day where the line between the living and the transitioned is at its weakest point. That’s why it was at one time known as The Day of the Dead in Pagan lore
It has morphed into something different but still maintains some of the pagan rites. As with most holidays, it seems anathema to say HAPPY before ANY somber holiday. And Halloween certainly fits that mold.
Wikipedia has this to say about the holiday:
Be Happy! Be Well! Be Positive!
Blessings to you.
Once you realize that life is eternal,
That our souls our eternal,
That we return to light and physical over and over;
We then lose all our distress
We then lose all our fear of dying. For there truly is no end.