Disneyland is not going to be closed on July 17th...baring some natural disaster or other unforeseen event. It was not closed on July 17, 2005 either.
DisneyGeek.com has photos from July 17, 2005 and several Dis'ers on this board were there as well.
Now...it's possible that Disneyland might reach capacity at some point during the day and then have to stop allowing guests to enter the park. That has happened in the past, but full park closures to the public have been very rare (e.g., the day of mourning for President Kennedy, 9/11, after the Northridge Earthquake for safety inspections).