Be warned that this guide contains minor spoilers for the Dead Money DLC. This is a story related mission that takes place after finding all 3 companions and returning to the fountain at the Villa.
You'll receive three new missions (Fires in the Sky, Strike Up the Band, and Mixed Signals) that involve escorting each companion to a switching station in order to simultaneously trigger the Gala Event, and open the casino's gates.
Fires in the Sky has you taking Dog and God to Salida del Sol South. Navigate the streets, destroying ghost people as you move along and be sure to engage Dog in conversation to unlock the Ghost Hunter perk, making it easier to destroy them outright. Either reason with God to stay at the station, kill two ghost people for meat to appease Dog, or lock the cage to force God to activate the switches. Any choice will finish the mission.
Strike Up the Band will take you to Puesta del Sol with Dean Domino. His perk will help you avoid damage from The Cloud that blocks your path in many areas. Travel through the Ruined Cafe and when you reach the rooftop with the sparking wire, either threaten him to stay and complete the circuit or activate two security holograms to protect him. When he finally agrees to preform the task, the mission is finished.
Mixed signals takes you and Christine to Puesta del Sol, and a switching station rife with speakers and The Cloud. Your companions perk will help lengthen the time you have to navigate around speakers interfering with your bomb collar. A fuse box is in need of repair at the switching station, and either 3 fuses or a repair of 60 will get it working order again. You'll also need to activate a terminal by the elevator and then engage Christine in conversation again. After she agrees to stay the mission will end.
Now you have to proceed to the bell tower in Salida del Sol and trigger the Gala Event. Numerous ghost people and speakers will greet you along the way. Be sure to grab the Super Stimpak recipe on the ledge, and the Holorifle mod recipe when you reach the top.
Pull the switch and fireworks will explode over the casino. Ghost People will now emerge in the courtyard below, and you'll have to fight or flee in order to backtrack back to the fountain and continue into the Sierra Madre casino.