oment to see where I'll be invited?
Too many potlucks,
As I've talked about before, a lot of organizations in Philly host Shabbat dinners, and sometimes they conflict with each other, but organizations typically have their schedules publicized well in advance, so you can know what's coming up and decide where you want to eat. When an organization is hosting a potluck, I think it's ok to RSVP as late as Friday afternoon, and, if necessary, to cancel as late as Friday afternoon. There are some important exceptions, though: If the potluck has a capped number, which home-hosted meals sometimes do, then don't expect to be able to reply on Friday and still have a spot waiting for you. Also, if you've told the host what you're going to bring, backing out too close to Shabbat means that the meal won't be complete without your contribution, in which case, you should find a way to drop off food even if you decide not to attend.
When an organization has a Shabbat meal that's not a potluck, the rules are different. If there's an RSVP date listed, respect that deadline. Caterers need headcounts in advance, and even if it's a meal that someone in the community is making and hosting for an organization, people need time to shop and plan. While I always say, "there's always room for one more," and I always want to mean it, sometimes making that room is really inconvenient, and, truly, in some situations there just isn't the space or the food to accommodate last minute guests. If you need to cancel your reservation to a catered meal, don't expect a refund, since the food has already been ordered and paid for by the organization. If for whatever reason, you're not going to show up somewhere where you're expected, be sure to send an email letting the host know, and send it before sundown.
In terms of personal invitations, the etiquette changes yet again. We'll use a "half-life" rule here: Take the amount of time between when you receive the invitation and the date of the meal, and reply no later than the halfway point between the two dates. If you get invited two weeks before, reply by the Shabbat in-between. If you get invited on Wednesday for dinner on Friday, reply by Thursday evening. If someone is inviting you to a meal two weeks in advance, it's probably because he or she needs that much time to plan, and you can be a good guest by replying accordingly. On the flip side, I can certainly say from personal experience that if I'm inviting you on Wednesday or Thursday to a meal on Friday or Saturday, I'm probably going to be throwing things together at the last minute anyway, and if I haven't planned far enough in advance to invite you before Wednesday, I probably haven't shopped yet, either.
Once you have accepted a personal invitation to a Shabbat meal, you cannot cancel because a better option comes along. I mean, you can, but you really, really shouldn't, especially not if you ever want an invitation again. If you receive a more appealing invitation after you've already agreed to go elsewhere, you can say, "Thanks so much for thinking of me. I'd love to be there, but I have a prior engagement. Would it be possible for me to drop by for dessert? If not, I hope I can join you another time."
Finally, in the midst of all these invitations, I hope you are reciprocating and inviting people to your home for meals, too. Hosting a Shabbat meal can be an incredibly gratifying experience, and the best way to ensure that you continue to receive invitations is to extend some invitations yourself.