Of American-English origin, the expression dadchelor party denotes a party given for a man who is about to become a father, attended by men only.

In this expression, the noun dadchelor is a blend of:
– the noun dad, an informal word for father,
– the noun bachelor as used in the expression bachelor party, denoting a party given for a man who is about to get married, attended by men only.

These are, in chronological order, the earliest occurrences of the expression dadchelor party that I have found:

1-: From Under the Dome: Highlights from The Examiner’s City Hall blog, published in The Examiner (San Francisco, California) of Thursday 10^th September 2009:

Friends throw mayor ‘Dadchelor’ party
Mayor Gavin Newsom was treated to a “Dadchelor” party Tuesday evening at the Giants game at AT&T Park, aides said.
Such a party is a celebration for soon-to-be fathers. Newsom’s wife, Jennifer Siebel Newsom, is pregnant with the couple’s first child, a girl, and is due Sept. 24.
Fifteen friends attended the game with Newsom, cheering from box seats and eating popcorn and drinking sodas, said spokesman Nathan Ballard, who attended the party.
The celebration was put together by the mayor’s brother-in-law, Geoff Callan, Ballard said.

2-: From the Great Falls Tribune (Great Falls, Montana) of Friday 22^nd July 2011:

It’s buddies, beers and diapers to welcome baby
By Aimie Tjader
McClatchy Newspapers

Minneapolis—Vera Becker, six months pregnant, is starting to picture her husband drinking beer, playing yard games and smoking cigars in the backyard with his friends. But she’s not mad about it. She’s all for it.
The Minneapolis father of one, with another on the way, is making plans to have a “diaper party,” a male version of a baby shower, where guys get together for one last hurrah before the new baby arrives. In this version of the celebration, each guy brings a package of diapers for the dad-to-be.
[…]
Sometimes called a dadchelor party, man shower or daddymoon, diaper parties are the latest way to acknowledge a baby’s arrival.
Some guys choose bachelor-type weekend getaways to Las Vegas, while others take a more subdued approach, exchanging parenting advice over burgers and beer.
[…]
The rising popularity of such celebrations reflects the bigger role that dads play at home, parenting experts say. Fathers today are more actively involved in day-to-day child care, and more men are staying home, while women go to work.

3-: From The Tennessean (Nashville, Tennessee) of Sunday 11^th March 2012:

Throw a ‘dadchelor’ party
Fete the father-to-be with a baby shower just for him
By Andrea Pyros
www.RetailMeNot.com (MCT)

You’re about to have a baby, so of course you’ll want to gather your closest friends for a bonding, talking-about-kids and blowing-off-steam get-together before you’re busy for the next, oh, 18 years or so.
Sounds like a baby shower, right? Almost, but it’s for expecting dads-to-be, and it’s called a “dadchelor” party, otherwise known as a man shower. According to popular website TheBump.com (www.thebump.com), one in five dudes have either been to a man shower or had one for themselves. To find out more about the how, who, when and what, we talked to Elena Mauer, deputy editor of TheBump.com.
‘Hanging out with friends’
Unlike baby showers, which often seem similar from one to the next (eat brunch, open presents, play a few silly games, share stories of the horrors of childbirth), there seems to be no one way to have a dadchelor party, says our expert. However, the common theme is that a soon-to-be papa spends some time with close pals while doing something he enjoys.
Mauer has heard of a variety of events: backyard barbecues for a handful of friends, a few people getting together to play video games or poker, a group fishing or golf trip and in a few instances, a huge “all-out trip to Mexico or Vegas.” A popular party centerpiece she’s seen? A diaper keg, which entails the dad-to-be buying a bunch of beer, and all his male friends coming over with Huggies or Pampers for presents. Cute, right?
The evolving roles of fathers
Yes, half of the couple is carrying a basketball in her stomach for nine months and then pushing it out of her body, and the other isn’t. But Mauer says that it’s still totally reasonable for men to want to celebrate upcoming parenthood, too. After all, she points out, “Dads are so involved with pregnancy and parenting” every step of the way these days.
Mauer posits that the growing popularity of dadchelor parties may be due in part to the “babymoon” trend in which a couple enjoys some time together during the woman’s second trimester. Men are now aware that they can use this time to get together for a last hurrah with friends, too. “Guys realize their lives are going to change when the baby comes, so why not have a little fun beforehand?” she explains.
There’s also the celebrity influence to consider. Mauer says that when Kourtney Kardashian ¹ was expecting her first child with boyfriend Scott Disick ², he went out for a night of male bonding in Miami, accompanied by friends like Kevin Federline ³ and Shwayze ⁴. The group had a steak dinner, went clubbing and apparently, “Kevin Federline gave some daddy advice” to Scott. And viewers who tuned in to “The Rachel Zoe Project” ⁵ on Bravo saw Rachel’s husband, Rodger, having his own man shower in Vegas.
It’s not for everyone, but for many men, a dadchelor party can be a fun and special way to acknowledge the big change coming their way.

¹ Kourtney Kardashian (born 1979) is a U.S. media personality, socialite and fashion model.
² Scott Disick (born 1983) is a U.S. media personality and socialite.
³ Kevin Federline (born 1978) is a U.S. rapper, DJ, actor, television personality and fashion model.
⁴ Shwayze (Aaron Smith – born 1985) is a U.S. rapper.
⁵ The Rachel Zoe Project is a U.S. reality documentary series starring the U.S. fashion designer Rachel Zoe (born 1971).