How do guests interpret "Please reply by [insert date here]"?

They need to have the response back to you by April 5

I would go with the second one that you suggested. They'll try to get it in before the deadline and not ON or AFTER the deadline.

Some may think of it the first way and others the second way--- but normally it's to be the second way, to respond back to you by the date.

Well if they're anything like the guests invited to my bridal shower, it means completely ignore the RSVP date then text the bride (me), who wasn't listed on the invitation, two days before the shower.

Personally, unless I am trying to work out logistics I try to respond right away. Some people might not know what their schedules are like a couple months in advance. I would give it a few days after the respond-by date to allow for USPS then call the stragglers.

I would plan as if people think it has to be postmarked by that date. Build in a few days wiggle room to accomodate the time in the mail and then start following up.

People are natural procrastinators. I would expect that most people would have the RSVP in the mail by April 5, not in your hands by then

So, it MEANS that you need the reply by the 5th, but a lot of people will probably try to get it in the mail by the 5th (so you won't get it for a few days).

If you're that worried about how people will interpret it, make the RSVP date the week before.

I would interpret that as it needs to be put in the mail/postmarked by that date. Although I do think it's a bit rude to wait until the last minute to reply. All you're asking is for them to check a box and take a 30 foot walk to their mailbox!

If people can wait until April 15th to postmark their taxes, you can bet your ass they will take it as it needs to be mailed by your RSVP date. Sucks, but it's what it is.

It would be interpreted by many guests as, "Either respond as soon as I get the invitation, or put it in a drawer and forget all about it until the bride calls to ask if I'm coming or not." We got almost all of our RSVPs either within the week we sent it out, or the day of the deadline.

If they read it at all, they interpret it as, "Blah, blah, blah, blah, blah...." IMO.