Clash-of-clans – How to get a Santa Strike

I just triggered it in one of my attacks! It is [holiday] trap, hidden until triggered by one of your troops. It will proceed to pop open, pop red smoke, then Santa will carpet bomb the area with presents - which are bombs.

I was able to capture some screen shots below of my reply

Troops triggering the trap:

Trap popping open (didn't seem like any damage was done here):

Red smoke, followed by Santa flying above it:

Present Carpet Bomb!

[AoE] Explosions!

Not much left (but unsure of total damage in AoE): There is no damage to attacking troops. The "trap" just drops presents with 5k elixir each. I was attacked recently and the archer that triggered it didn't die. Santa just flew by and drop 5 presents with 5k elixir a piece.

If you are on the receiving end of the attack and your Santa Strike is triggered, I found that it will leave presents for you to open near that attack. I gained 5,000 elixor per present.

It depends on the building in question and your town hall level.

For instance, for Cannons, you can build 2 until town hall 5, when you can build a third. Town hall 7 adds 2 more for a total of 5, and you get one more at town hall 10 for a total of 6.

There is a table on this Wikia page which lists the buildings available by town hall level.