I hate these types of promo -- you only find out until you get there after swiping your card on the Kiosk. However, the 15X will generally be given to the elite players.
Here's the verbiage on their website:
"Thursdays in May
Earn Rewards Faster than Ever!
5-15X POINTS - May 3, 10, 17, 24, 31
All Rewards Club members get 5-15X Points every Thursdays in May.
Swipe your Rewards Card any time after 4 am at any Fortune's Rewards Kiosk to reveal your Mystery Multiplier.
Use your points for free play, free meals and free merchandise!"
Good luck.