First, you have to live out in rural Idaho and have TONS of space, parking for 30 cars and a big yard, house and shop.
Then, you need to have a couple of friends from church who are also turning 40 the exact same week you are so you have an excuse for a giant party. Set it for a Saturday afternoon so everyone is free to come.
To get the word out, you need to send invites to practically every living relative of each of the three birthday people, everyone in your church, every work acquaintance and then everyone else who lives in Moscow. Also, set up a website with info and directions to your very rural house.
On the day of the party, disregard the impending rain and set up lawn croquet, a giant trampoline and several patio firepits. Set out drinks everywhere like pop, bottled water, beer, wine and juice boxes for the kids.
In the house have piles of toys for every age of kids and snacks for everyone. For dinner barbecue brisket and serve a chocolate-wine reduction for the sauce. Have the guests bring the rest of the food so there is a giant spread and let everyone partake freely.
After dinner set up a movie in the shop and entertain fifteen kids for an hour and a half. Allow the adults to chat, to roast marshmallows and play board games.
Then hold a gourmet chocolate tasting and blind wine tasting. Shortly after that create a ginormous bonfire and set off some leftover fireworks. Enjoy the fact that during the party everyone was so laid back that no one got upset and everyone enjoyed each other's company and even each other's kids, since almost everyone there had kids of their own. Also of note was the fact that everyone who enjoyed alcohol drank responsibly, enjoying the taste not trying to get plastered. People helped with each other's kids, chatted and got to know each other better and everyone had a wonderful time.
Yep, I'll go celebrate a birthday with these friends any time!