Organizers said representatives from nearly 200 agencies would attend.
The "LBI Thank You Fest" was to begin late Friday with a concert in Ship Bottom.
The main event was scheduled Saturday at the municipal complex in the Brant Beach section of Long Beach Township, where organizers said there would a day of celebration featuring music, food and other vendors, children's activities and arts and crafts. Celebrity chef Amanda Freitag, a judge on the Food Network's "Chopped," was to attend the first responders' barbecue.
The festival ends Sunday in Beach Haven with sidewalk sales.
Long Beach Township Mayor Joseph Mancini is urging everyone to come out to show their support to the New Jersey and out-of-state first responders who came to LBI's aid.
"They helped us out, and many of them had never seen LBI before the storm, so this is their chance to enjoy it," he told the Asbury Park Press.
"This is an event that we want them to remember," he said, "and we want them to know that we will always remember them."
He said more than 20 Louisiana state troopers have been flown in so they can attend.
Sandy, which struck Oct. 29, caused widespread damage on the 18-mile barrier island, particularly in the southernmost tip, where the Holgate section of Long Beach Township was devastated, and where rebuilding remains in progress. However, nearly the entire island is back in business for the tourism season.