Last-minute shoppers, be warned: you waste money picking up a six-pack or a bottle of wine on your way to a party. According to the shopper-rewards app Ibotta, consumers pay 9% more for beer on Saturday than on Monday and 6% more for wine than on Tuesday. This appears to be the first-ever data to show that by purchasing different products on different days, consumers can save cash.

“If you’re the kind of person who picks up on your way to a party you’re probably optimizing for convenience rather than value,” says Bryan Leach, founder and CEO of Ibotta, which is releasing a torrent of stats today to mark the $100 million worth of earnings it’s returned to its 18 million downloaders since launching in 2012.