Having a Ball

 is an achievement in the Fallout: New Vegas add-on Dead Money.

Requirements
Get to the bell tower to trigger the Gala Event that opens the gates to the Sierra Madre Hotel. You receive the achievement once you have activated the control pad that starts it. It is story related and cannot be missed. The achievement is worth 20 Gamerscore.

Walkthrough
After you have found all companions and returned to the central fountain, Father Elijah will tell you the only way to open and enter to casino is to activate the gala event. This will require you to do the 3 quests: Fires in the Sky, Strike Up the Band, Mixed Signals. This leaves your Companions at specific point across Sierra Madre. Once that has been completed the quest Trigger the Gala Event starts, for this you just have to activate the event via a console within the bell tower.