Every day, you can set up Pokemon that you want to wondertrade.
Then, at 22pm server time, the page will automatically trade your
Pokemon with set-up Pokemon from other players. You can set up a
shiny if you want, but it is very unlikely that you get a shiny
back. There's a special Shiny Wonder Trade, where you can set up
ONLY shinies and will only receive shinies :)
For the raffle, after the Wonder Trade happened, the one user per
day that won will receive the Code for Ho-Oh. If you win, you will
receive the code in your notifications. It will look like this: