Throwing a party together at the last minute is a very expensive and stressful way to do things. But it's situation normal in people who live chaotic, "spontaneous" lifestyles. I find that some folks live day to day or even minute to minute, and because their plans change without notice and they will not hold up their end of an agreement, it's absolutely impossible to make long-term plans involving them. The behavior is embedded so deeply it seems to be cultural, with each family having its own internal clock.
In my daughter's family, only people with grandchildren host, and the custom is to migrate around between various hosts within the same town, visiting each but not necessarily staying for long. The only time people host is for a holiday or a religious event, and the only acceptable structure is an open house wherein people drop in. They do not do "mealtimes". The concept of dinner being at a specific time is lost on them, but it doesn't matter because the service is buffet style and the kind of food prepared can safely sit and simmer for hours without the quality being affected. The rest of the time, they don't host: they go to restaurants. Trying to get one of them to commit to something like a dinner party is extremely difficult because they simply won't show up on time, and when they wander in 45 minutes after the food has been served, they bitch about how they don't like this, that, or the other thing. They believe the food is "bad" because after sitting and waiting 45 minutes it's become cold and unpalatable. Sometimes they arrive very early, or very late, or they don't show up at all. Overall, they can't be hosted at events that have a mealtime.
I once compared my father's family to a flock of birds: no discussion, no real communication, no central leader or organizer, and then without warning there's a general movement as though everyone is flapping off to the same location. It's extremely last-minute and higgledy-piggledy, which means that the celebrations tend to be small and local. On the plus side, the wedding receptions are cheap and easy-- cake and punch-- so people don't expect anything big or formal. There's an advantage to that from a frugality perspective. On the down side, it's very unfriendly to outsiders or to family members who live far away, because it's simply impossible to find out about an event in time to attend it. The lack of communication is so intense that it comes across as a deliberate attempt to make sure we don't show up, so that they can continue griping about how the excluded person doesn't show up or participate.
Then of course there's my mother's family: dates are announced several months in advance, and an official invitation must be sent out one month to two weeks before a major festivity. Everything is done in the home (yay frugality!). In my small home I can only handle a small party of 20 seated or 35 open-house. But to ensure that everyone has a seat and a meal, and to minimize waste, it's important to have a reliable head count. Ingredients have to be bought and made in advance to ensure that everyone's dietary needs are met. So the drop-in, "maybe" response to invitations is incredibly vexing.