It seems like the invitations went out a tad early, usually about three weeks is time enough for this kind of event. Probably most of the replies will trickle in as people firm up their plans over the next month.
In any case, I'm assuming you didn't put a reply-by date in the invitation? In that case, I'd agree that about 10 days before the event you could call those who did not reply with a reminder about the party and to confirm that they are attending.
I would not say anything about putting them as a "no" if they can't decide. A shower is a party for the benefit of the guest of honor and if someone decides at the last minute that they'd like to show up with a gift, they should be welcome with open arms.
If you're anxious to plan, then go ahead and assume all 17 will attend and plan accordingly. Probably the menu wouldn't be much different if you were going to serve 10 or 20, just the quantities. And you really don't need firm quantities till you're ready to buy the food, within a few days of the party. It's always a good idea to plan for more than are coming anyway, as you never know who might show up at the last minute or how much any individual might eat.
POD to all of this. I wouldn't be expecting RSVP's until 2 weeks before the event. I'd start calling 5 to 7 days before the event for anyone who hadn't RSVP'd. As far as wording, is "Hi, this is EtiqutteE, I'm finalizing our counts for the shower next Saturday. Are you going to be able to join us?"
Also, if I had invited 17, received yes responses for 15 and no for 2, I'd still have food and favors for 2 or 3 extras. You never know who is going to have a last minute "My mom decided to come into town this weekend but when I told her about the shower, she said she'd love to come and see Nancy. Would it be ok for her to join us?" or the day before one of the no's calls and says that their plans changed would it be ok if she joined you after all. Or even the GOH calling and saying "I just found out cousin Martha is in town, can we add her to the list?"
POD to you both.
I, too, understand that you really want to know ASAP so you can plan, and it isn't just the amount of food, either. I do agree that you should have plenty of food, and not just in case of an extra guest or two -- it looks skimpy to have EXACTLY enough food, unless I guess you are serving everything already plated to each guest. But I know that you need to know how many tables to set up, what table linens and dishes will work, etc.
But you don't need to know any of that yet. The time frames others have given are fine.
Remember that whether or not people respond promptly, they are your guests, and your duty of hospitality begins now, not the day of the party. So if and when you need to make a few phone calls or emails, be sure to do so graciously, not in a way that chastises or embarrasses them.
With all respect to Shoo, I disagree with this: "If they can't give you a definitive answer, simply tell them, 'Since we need to know right now if you are coming, I'm afraid we are going to have to put you down as a no. So sorry we will miss you.'" I would probably fume about it, but I'd simply keep them down as a maybe and plan as if they are coming.
In reality, do you REALLY "need to know right now" that they are coming if you call a week or so before the event? Probably not, and they know that, too, so it will just sound petty and punitive -- like, "Since you have no manners, you can't come to my party." I know that's not what you want. And there may be an excellent reason that they can't tell you yet, and they may say something like, "My mother is having surgery out of state two weeks before that; I probably will be back by then, but can I let you know a week before? I'd hate to miss it, but I don't know yet if my sister-in-law can spell me that second week." And suppose that that is an important (to the honoree) guest. How could you not agree to that very reasonable request?
Remember, this isn't about teaching them manners -- it's about having the people that your guest of honor wants there. Cutting out "maybes" affects her, too, not just them and you. It could even possibly affect other guests -- who might be counting on them for rides, or who might be embarrassed for them. You don't want to cast a shadow on your own party.
It really isn't a catastrophe to plan for, say, 20 guests and have 17 show up. Yes, they should all give definitive responses, and yes, they should do so promptly, and yes, if they don't, they are at fault, not you. But why let it diminish your party, especially as it's one you are giving for someone else?
So my answers to your direct questions are:
- yes, do proactively contact those who haven't responded
- timing: as lowspark and pame said. Basically, as late as you can
- wording: as politely and graciously as possible so as not to make them feel awkward, defensive, or shamed. You can even say you're having a problem with your email or you're not sure you have all the responses in one place or something like that -- i.e., pretend like it's YOUR fault you don't know. They know the truth and that you are just being nice. Like, say it's a shower for your niece, and the person you're calling is her mother-in-law that you don't even know. Why not give her that break?
- if people show up that you weren't expecting: well, there are people who will tell you to basically slam the door in their face. I mean, not literally -- they will say that you should regretfully inform them that as you weren't expecting them you are unable to accommodate them. I wouldn't do that. I would -- and have -- welcome them graciously (or as graciously as I can manage if they are really messing up my arrangements!) and treat them like any other guest. I'd do that any time -- it's not only a kindness to them, it's a kindness to your other guests who would be very embarrassed if you did that, especially if it's some friend of theirs. I mean, just picture how it would play out: you tell Aunt Pittypat you can't accommodate her. She is astonished but says only, "Oh! I see. I'm so sorry. Well, I certainly understand. I'll just come in and say hello to Petunia and give her this gift and then I'll be on my way." What are you going to do, not let her through the door? And then even if it goes as well as it possibly could, everyone else is going to be embarrassed, if not appalled, when you ask her to leave, or she says she's leaving and someone asks her why -- and that, my friend, will be everyone's enduring memory of your party. I know you don't want that!
This goes double for a party in honor of someone else. She will want to have those guests included. It would be very petty and selfish, in my opinion, to turn them away just to satisfy your own righteous indignation -- as justifiable and understandable as it would be!
That's just the way it is when you host a party. It feels like we will be happiest if we have everything under control. But the truth is that you will actually have a much more successful party if you are flexible, generous of spirit, and able to go with the flow. Certainly you will enjoy it more yourself.
It's very nice of you to host this shower! Have a wonderful time.