Mystery island - Pokemon Ruby
Feb 17th 2004, ID#1666 Mystery island
How to get o mystery island:
Your pokemon have these numbers randomly assigned to them everyday called something like break stat points.
If it happens that one of your pokemon have a break stat number of 0, the island appears.
It's only there for a day because the numbers for your pokemon change everyday.
Also, the number changes when you deposit the pokemon in the box, so you can try doing that.