The 84-year-old Playboy - married twice before - announced his engagement to 24-year-old Harris on Christmas Day.
"It's official: Crystal & I will be married on Saturday, June 18 at the Playboy Mansion," he wrote on Twitter. "Happy days!"
Also confirming the news, Harris simply tweeted: "6.18.11".
The couple made the announcement as they carried out a joint interview with Piers Morgan on Piers Morgan Tonight.
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