Probability of Mystery Gifting

To begin, I'll tell a story. Skip to the question at the bottom if you have no interest. I was playing ARAM as Soraka. I did quite horribly, but I healed the hell out of my team. I often died to the global ults from Lux and Ezreal. I played really passive at first. I stayed in the back and diligently healed my teammates. However, I decided to flash and use my combo to secure a kill just to laugh a bit. The Yi, who dove non-stop, encouraged the behavior and for the rest of the match I was playing aggressively. He gifted me a skin in the post-game screen and wished me a happy holiday. Somehow, I ended up with a legacy skin from '09.

My question is, what are the chances of getting a legacy skin from the mystery gifting system?