"The assessment is based on terrorist interrogations and the police’s investigation of the incident, among other things, which reveal that the terrorists intended to infiltrate Re’im and other kibbutzim near the Gaza border.
According to a police source, the investigation also indicates that an IDF combat helicopter that arrived to the scene and fired at terrorists there apparently also hit some festival participants."
It was planned and begun before the attack. It had the bad luck to be in the wrong place at the wrong time when the attack started.
Yup, according to Wikipedia the location was proginally in southern Israel and moved two days before the festival. Hamas’ attack planning wouldn’t have included the festival.