We all know that Santa Claus is responsible for the huge undertaking of making and then distributing toys all across the globe to rich Christian children (despite the propaganda perpetuated in such made-for-tv documentaries as "Santa Claus Is Coming To Town," poor and / or non-Christian kids don't generally get toys for Christmas).
To assist him with these duties, Santa has hired vast legions of elves. They help him keep lists of the naughty and nice kids, make the toys, and wrap and package them for delivery. They also take care of the reindeer.
Santa has angels working for him, too. You've probably seen some of their work at the local mall, in the form of the Christmas Angel program, by which Santa attempts to distribute some Christmas loot to poor kids in rich Christian countries and support the merchants and economy at the same time. The Angels have other tasks, too, such as supervising the elves in the toy shop and helping Santa with any shopping he must do (Santa is generally too busy taking orders from kids at the mall to do the shopping himself). Sometimes they help Mrs. Claus with the bookkeeping, too.
In addition to supervising the whole enterprise, hiring and firing elves and angels, selecting the reindeer for the annual Christmas ride, and predicting what gift will be this year's "hot ticket item," Santa historically has handled the task of selecting, cutting, and trimming the Christmas tree for his annual Christmas party for the elves and angels. This is a huge and important task, as the holiday party represents Claus, Inc., to workers and customers alike. The tree must be perfect, and the decorations must also be perfect.
(Of course, just like any corporate holiday party, the party is, at best, tolerated by the worker elves and angels. They'd really rather be at home with their own families and kids, but they show up at the party anyway to keep ol' Santa happy, because jobs are scarce at the North Pole. Santa is pretty much the only game in town.)
One year, Santa had other pressing items on his "to do" list, so he delegated the task of selecting and trimming the tree for the party. He asked his best worker angel, Beatrice, to handle the task. He explained that the tree must be perfect, that it must be placed in exactly the right spot, and that it must be decorated perfectly -- all by 6 p.m., because the party was to start at 7:00. She had enough to do already and was a little miffed, but she really had no choice. After all, jobs are scarce at the North Pole, and she had a family to feed.
So Beatrice went out into the extremely cold North Pole day, which at that time of year was really more like twilight and lasted only a couple of hours, and after much deliberation, she selected a fine evergreen tree for Santa. She paid (from her own pocket!) a local elf to cut it down and haul it back to Santa's lodge where the party would be held. But then she faced a problem. Every year, Santa placed the tree in a different location. She did not know where he wanted it that year and he had told her the location must be "exactly right."
So she went looking for Santa, to ask him where it should be placed. Santa was... uh... shall we say, "indisposed." He was ... uh, busy... with Mrs. Claus. Nevertheless, Beatrice knocked on Santa's door and asked, "Santa? Are you in there?"
Santa ignored her and kept on with his, uh ... work.
She knocked again: "Santa?!? Are you in there?!?"
He ignored her again.
Beatrice was nothing if not persistent (not to mention running short on time), so she knocked again: "Santa?!? I really need to talk to you. Where do you want me to put this tree?!?"
Santa, frustrated with this interruption of his very important, uh... work... shouted back: "I'll tell you where to put it, just shove it up your @$$!!"
And that, my friends, is how the tradition began of placing a Christmas Angel at the top of the Christmas tree.