Palestine

“Around 6:30 in the morning we started hearing explosions,” Gaster says. ... Gaster says that at this point, festival security advised everyone to get down on the floor and put their hands above their heads for protection. But after 5-10 minutes, Gaster says, “the policemen shouted in the microphones, ‘Okay, get in your cars and go.'” ... “The moment the policemen said ‘go now,’ I ran,” Gaster recalls. “I didn’t wait, because we know it’s a rocket attack. You need to act quick.” ... By the time Gaster and the others made it to a villa rented by the production team, located approximately 30 kilometers away from the festival, they had started getting texts and phone calls telling them that minutes after they drove away from the site, Hamas fighters had arrived “with machine guns, with RPGs, with grenades, and just slaughtered whoever they could.” ... Gaster and those he was with turned the villa into a command center, contacting IDF, other Israeli security services and “all of our friends that we know personally that have firearms that have connections that can go there.” ... Gaster says it took IDF and special forces a few hours to arrive on site, with those who were there attempting to defend themselves in the meantime. “At the party there was already a police force, like any licensed party,” Gaster says, “and they were the first ones to try to give assistance by fighting… We are Israelites, so most of us have military experience, and a few from the production managed to kill some terrorists with their bare hands and their weapons.” Gaster says a person from the festival killed two of the attackers after taking their guns. Gaster says he and the team at the production villa were being sent on-site locations from various attendees and then sending these locations to this person, who then went to help these attendees. ... While Gaster had just arrived to his home in the north of Israel when Billboard spoke with him at around 1 a.m. local Israeli time (he says the IDF controls most of the area between where he was and where he lives, so he felt safe to drive home), he says that amid the chaos they are all “still trying to find any signs of life.”

Raz Gaster was an Artist Manager for the festival. So in summary:

• armed police (most with military experience) were present from the start of the music festival and were the first Israeli force to fight against Hamas militants during the attack
• a few minutes after the rocket attack, the festival's production team sped to a villa they'd rented, located 30km away from the party and in territory controlled by the Israeli army
• the production team turned the villa into a "command center" to mobilise Israeli soldiers, security services, and armed Israeli citizens towards the festival site Hamas was occupying
• members of the production team were armed, and killed Hamas militants "with their bare hands" (?) and with their guns
• one festivalgoer "killed two of the attackers after taking their guns" (how?)
• various attendees conveyed locations (of militants?) to the production team at their "command center", which they then relayed to this armed individual (who already killed 2 Hamas militants) to help those attendees who conveyed the locations
• Gaster then drove from their command center, to his home in the north of Israel the following night/early morning, and was interviewed by the media

Source: https://www.billboard.com/music/music-news/israel-music-festival-attack-artist-manager-account-massacre-1235436829/

It's hard to know what's real but I saw a video of soldiers ducking and diving between the festival goers while shooting at Hamas.

They were then joined by an Israeli Merkava tank. Equipped with a powerful 120 mm cannon, this tank was of no use at the time, except as a refuge thanks to its armored plating. Dozens of participants took cover behind it, along with some of the police. Bullets rattled. The assailants were there.

The article seems to say that the tank was there before Hamas. So maybe the ravers were caught in the crossfire.

From the preview:

People were smiling. Videos taken by those attending captured the scene. At around 6 am, in the distance, plumes of smoke appeared in the sky. Rockets that were fired from the Gaza Strip were intercepted by Israel's Iron Dome, its anti-aircraft system.

How much of a warning do you need?

Here you go