Ariel Sharon served as the Israeli Prime Minister from March 2001 to April 2006. Soon after 9/11, Sharon was named in a story entitled "4000 Israeli Employees in WTC Absent the Day of the Attack", which looked to claim Israeli foreknowledge of the attacks. Though not necessarily Sharon directly:
But what festival? Which organisations? When was he travelling? This all seems a little vague. And when you look into it, a little inaccurate. There was an event Sharon was supposed to be attending, held in New York, by the General Assembly of the United Jewish Communities, however this was to be held on the 23rd of September, not the 11th.
It seems the rally could have been held in June (safely), but was only put off until September by Sharon not able to guarantee he'd be there. This seems an odd kind of foreknowledge.
What's more, the rally itself was understandably cancelled on September 12th.
It's possible that after the attacks, but before the official cancellation, the Israeli Secret Service recommended Sharon shouldn't attend (it would be hard to imagine them doing anything less). Therefore it may be true that Sharon cancelled travel plans on September 11th, but this is in no sense mysterious, and doesn't require any foreknowledge at all.
See this page for more details on the Sharon claim and other Israel-related conspiracy theories.