This is probably a question for Miss Manners, but she won’t return my calls. Why bother with save the date cards? If you know you’ll have an event, when it will be and who you’re inviting, why not just send them invitations? Why send a card that essentially says “you’re not invited yet but you will be”?
Maybe you’ve chosen the date, but not the time or location yet.
Several reasons:
The date might be fixed but the venue isn’t.
(For weddings) you haven’t finalised numbers so you don’t know yet who will be invited to the whole thing and who will be coming to the evening.
Wedding invitations are usually sent out quite close to the event (like, within a few weeks), along with specific instructions, directions, gift list (yes I know it’s not The Done Thing but everyone does it) etc. All that stuff won’t be sorted out until relatively late on, but you want to give guests plenty of warning so they can, well, save the date and get it in their calendar early.
The benefit-of-the-doubt part of my brain says save-the-dates are something you buy immediately after you actually SET the date, to give everyone plenty of time to clear their schedules.
The cynical part of my brain says that stationers saw a chance to milk even more money out of crazy-on-wedding-adrenaline brides.
I suspect that both of these are true.
A couple of friends just got hitched (huzzah!) - and since they live right below me in our apartment building, I got to see a lot of the pre-wedding crazy up close. 
For almost everyone, weddings are the biggest logistical mess we’ll ever be involved with in our lives. Everything costs money, and you never have enough seats for everyone you’d like to invite. The absolute worst-case scenario is for people to say they’ll show up, and then fail to do so; you’ve wasted money on their seat, and denied someone else an invitation who would have been delighted to show up. It probably costs less than a single seat at your wedding reception to be absolutely, comprehensively, repetitively crystal-clear about when your wedding will be - so it makes very good sense to do that.
For a wedding, you want to let people know relatively early when it’s going to be so that they can keep it in mind as they plan and schedule. You don’t want their first inkling come when you send the invitation a month or whenever before hand because they might already have committed to dog-sitting for a friend or whatever. You don’t want to send the invitation itself months early, though, because it’s unreasonable for people to respond that far in advance. What if they’re just a coworker and they’ve said they’ll come to your wedding but then find out later a family member’s wedding is the same day? Now they have to go through the hassle of letting you know they’re not coming to yours. If they do the reasonable thing and wait 'til closer to the wedding to send in the invitation, there’s a decent chance it’ll get lost.
Note that whenever I said “you” I meant “me”. It was just easier to write that way.
So, if I get a save the date card with all the pertinent information, it’s happened more than once, I can assume it’s because the host is a bit drunk with the excitment of it all and has dispatched with common sense in favor of partaking in every indulgence?
They’re a way of advising people of when an event will be without requiring a response regarding attendance yet.
There are conflicting demands here:
[ul]
[li]It’s a courtesy to invitees to let them know about the event as early as you can; and[/li][li]It’s also a courtesy to invitees to request that they commit to coming or not as late as practical. You eventally need a firm head count for planning, but generally not six months in advance.[/li][/ul]A single communication, for events planned on timescales like weddings, may have trouble fulfilling both of those courtesies. You’re either springing the invitation on relatively short notice (which may mean people who would have preferred to come have other commitments), or you’re requesting an RSVP absurdly early.
Well, what prompted the question was getting a save the date with all the details followed by the actual invitation about two weeks later. (It was a bit of a rush wedding.) That basically made the save the date an informal version of the invitation and didn’t really accomplish much by way of providing advance notice. It sounds like I just need to get smarter friends.
non-wedding: I work for a charity and we send out save the date cards strictly for advertisement and word-of-mouth. They’re also a little easier to distribute than an actual invitation, and cheaper to print, usually.
brad_d’s explanation was pretty good.
When I got married, we sent out “save the date” cards in December (we enclosed them in Christmas cards). The wedding wasn’t until the following August, and the actual invitations didn’t go out until about 8 weeks before, I think.
Huh. I’ve honestly never seen it. I’ve been invited to a number of weddings, and I’ve never seen anything regarding gifts on the invitation (except for one that said “we only request the gift of your presence,” which stuck out to me, since any mention of gifts at all is incredibly unusual to me.) The most is maybe there’s a wedding website with registry information on it, but that’s it.
No one I know puts gift registries in invitations either. I don’t know who these “everyones” are but they obviously need a lesson in manners. As I write this, I do recall the second wife of a physician friend of mine putting them in. The ex wife had a ball with that little faux pas.
The last “save the date” card I got was for a nephew’s confirmation. Because of the large number of confirmands, the church was going to have several ceremonies on the same day, and they hadn’t yet finished the lists of who would be confirmed when. That also meant the time and venue for the party afterwards was still undecided. However, it was getting close enough that the young candidate’s mother wanted to tell everyone when it would be. Hence, a save-the-date card, followed by an invitation when the church finally got the details worked out.
Colophon is in England, I believe, so customs there may vary from the US.
If you ask for a commitment too far in advance (e.g., 6 months), you’re going to get a much higher flake factor than if you ask 6 weeks in advance.
Any mention of gifts at all is extremely tacky.
The way my friends did it was to give their wedding website URL on the invitation, and the website had a link to their online gift registries. Seemed tasteful enough.
We sent save-the-date cards for our wedding because we have relatives that are very far away – some on the other side of the planet, in fact – so we wanted to give them plenty of advance notice so that they could book their travel, if they wanted to attend. The actual invitations were sent out only a month or so beforehand, as normal.