It's basically an open invite for a group of friends, and a standing thing that just happens. If you have no other plans, you basically go. Sometimes I have to drag myself out of the house to do it - but it's worth it every time. The group that attends sometimes is just one other couple, sometimes it's a house full of 30 people if everyone comes, brings their kids, and a few friends of friends tag along.
Definitely one of the better things I've been involved in!
I am retired now and lucky to have my (now adult) children living nearby. Every Friday I cook a large pizza dinner for everyone, their spouses, and their children, and any friends they want to invite. I will always be an introvert, but this weekly meal is the highlight of my week.
The difference between this and what my parents did is that I host for my own pleasure and for what it does to bind the family together. I don't expect reciprocal invitations, and no obligation is ever implied (I hope).
Hopefully it helps the next generation stay connected and maybe your kids pick up this habit when older and it becomes a tradition.
TBH, if I were to retire today, I'd love to do that, too. I only have one son, so probably it's not going to be a big dinner, but I'd also invite friends of the family, too.
My grandma had four kids who all married and gave her ten nephews in total. On new year's eve we had this large dinners where pretty much 20 people gathered (sometimes more, when nephews got old enough to bring their partner OR some other uncle/aunt from other sides of the family joined).