As for as I know, they haven't advertised or started promoting the 20th anniversary show so nothing is official yet. However, on the WWE RAW wiki page, which isn't totally reliable, it does state the 20th anniversary show is on Jan 7th so take it for what its worth.
Isn't it weird how the 15th anniversary was in December 07 BTW?
Also, Rock visits this place whenever he's on vacation.
y
http://en.wikipedia.org/wiki/Goat_Rock_Beach