So, when should I ask my guests to reply by? Short answer: 6 weeks prior to your wedding date. Why?
Set the RSVP deadline two weeks before you actually need the responses to arrive. Most guests will feel good if their response is simply postmarked by the reply date, as the taxman has trained us to do… really, 4 weeks is sufficient however the extra 2 weeks will allow you the extra time to track them down to get their response!
If you’re using mail RSVP cards, make sure you include a pre-addressed and pre-stamped return envelope with adequate postage. Hint: do not pre stamp RSVP envelopes going out of the country! They will need to stamp their own to send it back to Canada!
Another helpful tip - create a list of all of your guests (you probably already have one made in excel!) and give each guest or family a number. Write the number (very small) on the back of the RSVP card before sending it out. When you receive your RSVP's back in the mail, and you cannot read uncle John's handwriting or someone simply didn't bother to put their name... (who is this?!) you can turn the card around and know exactly who it came from!
Lastly.. PLAN AHEAD!
Don’t wait until three days before your catering numbers or seating chart spreadsheet is due to start calling everyone in panic. Add some calendar invites to remind yourself when to send the first gentle reminder and when to start picking up the phone and dialing. Another sanity-saver is to build in some buffer for the unexpected yay or nay. Expect one or two guests to drop out at the last minute and one or two others to call at last minute and ask whether they can still come. If you are mentally and logistically prepared for this to happen, it will be less stressful when it does!