The new Us Weekly (on newsstands today) reveals shocking new details and exclusive pictures of Jon Gosselin and his mistress, as the father of 8 cheated on Kate's 34th birthday.

On that day, March 28, Jon was on a steamy, booze-filed trip to Park City, Utah, with 23-year-old school teacher Deanna Hummel (the June 1 Jon & Kate Plus 8 episode only showed him helping handicapped children ski as Kate celebrated her birthday alone with the kids on a "surprise" trip to the Ace of Cakes' kitchen).

See more controversial photos of Jon and 23-year-old Deanna Hummel

Deanna had joined Jon at the annual Pond Skimming race at the Spring Gruv festival in Park City. (Of his trips to Utah, Jon told a local paper: "I have eight kids, two dogs and live in Pennsylvania, and I can still hang here with 15-year-olds.")

See the Gosselin family through the years

"Hey, it's Kate's birthday," Jon told a local friend, the new Us Weekly reports. "I should get her something." Despite visiting a jewelry store, the pal says Jon never followed through. Instead, he spent the day downing free Budweisers with Deanna at the event.

That night, a source says Jon and Deanna "curled up" at the Downstairs bar (co-owned by actors Chris and Danny Masterson), adding, "It was snuggle-o-matic."

See photos of reality love gone bad

As the night wore on, Jon and his hard-partying pals began "reeking of pot. I know that the kids he's hanging out with like a little herbal substance."

See what Kate Gosselin -- and other stars -- look like in a bikini

Jon and Deanna first met while partying at local bar Chill Lounge near his family's $1.1 million home in Wernersville, Pa. "Jon was a dirtball," Elyse Motague, the waitress Jon asked to have send Hummel a shot by way of introduction, tells the new Us Weekly.

See what Kate Gosselin looked like in high school

For more on Jon's seedy behavior - including how he spent Valentine's Day buying shots for girls, TLC's staged "romantic" getaway that got canceled last minute because of Jon's affair, and Jon's appalling treatment of a babysitter - pick up the new Us Weekly today!

Get more Us! Follow us on Twitter, Friend us on Facebook, Join Us Weekly's Celebrity Fantasy League, Subscribe to Us Weekly