Hugh Hefner's EX spends what should have been their wedding day at a pool party!!!

Crystal Harris spent what would have been her wedding day to Hugh Hefner by partying by the pool with Heidi Montag -- and Holly Madison isn't too happy about it!

"That's a new low!!" Madison wrote on her Twitter Saturday as photos surfaced of Hef's runaway bride reading her new Playboy by the Wet Republic pool. "That's disgusting and whoever booked her is tacky!"

When another Twitter user asked whether a reality TV crew would be following Harris, Madison added "trust me, NO ONE is interested in filming Crystal! She left Lifetime high and dry while filming a 2 hour special!"

Looks like Harris took notice, Tweeting "Hard times show your real friends..." to which Holly replied "No. They show HEF's real friends."

What do you think? Too soon to party? Sound off below!