The Event will be available from some time in December to sometime in January I believe.
You should be able to download it almost immediately, but you might need the PokeDex or the first Badge before you can do so.
After its downloaded, you should be able to receive it from any Pokemon Center, from a man who in Gen V stood to the left upon walking in.
That last one may have changed a tad, it could be a female, and she could be on the top or bottom floor this time. As nobody has downloaded an event yet in X or Y, we cannot be certain of where they stand and what they look like, but it shouldn't be a drastic change. --- Winner of ares9090's Slaking Award