Traditional etiquette is pretty spot-on about accepting social engagements in the first place. A quick rundown for those who aren’t familiar:
You get an invitation. For geeks, it probably comes in email, unless everyone has moved to Google Calendar without me looking. For big ticket events like weddings, you might still get a written invite. You reply by the same method you received the invite, unless another method is specified in the invitation itself.
You should reply to all personal invitations that come from people you know, either accepting or declining. A personal invitation is one-to-one (or one-to-a-few, in the case where households or partners are invited together). For public events like LUG meetings, you typically don’t reply unless there’s specific instructions to, and usually those will ask for acceptances only. For those, general invitations are issued to the public, rather than specific invites to individuals. In case of doubt though, it doesn’t hurt to reply.
Responses should be timely and brief. Let’s look at those.
If the invitation has an RSVP date, this is the drop-dead date for responding. The date is typically influenced by things like the date on which your friend must tell their caterers the final numbers, or on which she wants to do that giant shopping run to buy all the pizza ingredients. Replying before the RSVP date is the best thing to do and you should aim to do this almost all of the time. If you can accept or decline right away, do that, so you don’t forget.
If you’ve missed the RSVP date by a few days you should typically send profuse apologies and, if you want to accept, non-pushy inquiries about whether a late acceptance is all right. If you’ve missed it by much more, you need to decline the invitation with profuse apologies for being so late. Accepting is no longer in the question, unless your friend tells you that you can do so. Don’t ask; if this offer is going to be made, they will make it.
If the invitation has no RSVP date, you reply as soon as you can make a decision. You can work out a rough drop-dead date, usually: when do they need to start spending money? For an average sized informal party, it’s probably a couple of days before. For a trip overseas, it’s probably several months before. You need to reply before you think they started spending money on guests.
Now, to your brief replies. If you’re accepting an invitation, you say something like “I’ll be there, and I’m really looking forward to it.” There’s special wording for replying to formal invites, basically mirroring the invitation back at them. (If they said “Ms Nerd requests the pleasure of Mr Geek’s company on the 9th June”, Mr Geek replies “Mr Geek accepts with pleasure Ms Nerd’s invitation for the 9th June”.) You likely only need this for weddings and there are lots of websites with full examples of how to word replies to formal invites. Otherwise, all you need to do is accept and express that you’re looking forward to it. Don’t go into any and all sacrifices you’re making to come. (“It’s really a pain to get flights that weekend, and my usual travel agent is
away, and I’m going to miss my new puppy, but I’m coming because I just love you that much.”)
Once you’ve accepted the invitation, you regard this as a fixed engagement and you must either turn up as you said you would, or break your word, a subject we’re going into soon. You never just fail to show up and don’t either warn them beforehand or apologise afterwards.
If you’re declining, the excuse you use in all circumstances is either “I’m so sorry, I have a prior engagement, I would have loved to be there” or “I’m so sorry, I won’t be in town, I would have loved to be there”. Not being in town gets its own excuse because ‘prior engagement’ refers to plans for a particular day. It just sounds weird to call your six month holiday overseas a ‘prior engagement’.
‘Prior engagement’ is what’s called a ‘polite fiction’: it covers everything from a real prior commitment to your need to wash your hair that night. That is, in the event that you can’t be bothered or just don’t want to, the phrase for this is still “I’m so sorry, I have a prior engagement.” (Alternative phrases include “I already have plans”.) Almost all explanation beyond that comes across more as “your event sounds dumb” than “I really wanted to come but can’t”.
One geekly explanation for this, if you like, is cognitive load. You care deeply about not liking smoky venues, or not liking events that Boring Dude is at. That’s fine, that’s why you’re allowed to decline invitations and organise your own events which are in fresh air and to which Boring Dude is not invited. There’s no reason to bring it up for a particular event, because that event is already being organised and there’s nothing that can be done about it without the organiser making radical changes, so you’re just adding to her load of things to fret about. If smoky venues and Boring Dude are about to cost the organiser your friendship, you should bring this up separately when a particular event isn’t under discussion.
The only exception to offering generic excuses is when invited to something by intimates who know what you’re doing most days: partners and very best friends. With them, you should be more open. Etiquette by and large is a guide to social relationships, not intimate ones.