Once upon a time, very long ago, Santa was having a very bad
Christmas.

The reindeers had gotten into the eggnog and got themselves quite
drunk. Rudolph's nose was bright red, and the reindeer were
tangled in their traces. Then as Santa was loading presents on
to the sleigh, the runner broke off one side, and dumped all the
presents into the snow. So, Santa called for the sleigh jack, and
jacked it up. Nailed the runner back on. About that time the
other runner fell off, and dumped the presents into the snow on
the other side.

About this time, the Christmas Angel came up to Santa and
reminded him that there was no Christmas tree at the North Pole
for Mrs. Claus. Santa had all he could handle, so he told the
Christmas Angel to just go out and find a good tree, and
everything would be OK.

There sat poor Santa with his head in his hands. No Christmas
tree... Not enough gifts for boys and girls... The reindeer in no
condition to pull the sleigh... and no sleigh to pull. Everything
had gone very wrong.

Suddenly, the Christmas angel returned dragging behind him a HUGE
Christmas tree."Hey Santa! I have your Christmas tree. Where do
you want me to stick it?"

....and so began the tradition of the angel atop the Christmas
tree.

(found on the rec.backcountry group)