The "Travel Tools" button is also at the very top, right-hand corner of the Southwest website. Click that button and then the "View All Travel Tools" option (at the bottom of the list) and you'll get to the page with the notice of when the next window will open.
First visit: Disneyland, July 17, 1955 (well, somebody had to be there on opening day!)
Most Recent Visit:
Disneyland - June 20-25, 2014
WDW - Sept 29 - Oct 14, 2014
WDW - late Aug, 2015