Saying no to a wedding invitation is a hard message. This is the harder one: you said yes. Months ago, easily, when the date was an abstraction. And now something has changed, money got tight, a work thing became immovable, your health, your family, or the honest quiet truth that you said yes in a wave of warmth you can no longer fund, and you have to convert a yes into a no after the caterer has your name.
Understand what changed when you RSVP'd, because it's why this message operates under different physics than a decline. A no at invitation time costs the couple nothing; they budgeted for no's. A withdrawal costs them concretely: a paid plate, a hole in a seating chart someone rebuilt twice, possibly a slot that could have gone to a cousin who wanted it. And it costs them something softer: your yes was, briefly, part of how they pictured the room. You're not declining an invitation anymore. You're taking something back. That doesn't make you a villain, life legitimately intervenes between RSVP and wedding, but it defines the job: the message has to acknowledge a real cost, not seek forgiveness for a preference.
Which yields rule one, and it towers over the others: speed is the entire ballgame. Every week between "I know I can't go" and "they know I can't go" converts a recoverable problem into an insult. Told six weeks out, the couple can adjust the count, reshuffle a table, maybe fill the seat, annoying, absorbable. Told six days out, they eat the plate and the gap. The message you're dreading gets objectively worse every day you dread it, which makes sitting on it the one unforgivable version of this. Send it the day you know.
The message itself, structured: the news up front, one honest line of reason, the cost acknowledged out loud, and a repair offered.
I have news I hate sending. I'm not going to be able to be at the wedding, and I'm so sorry to be telling you this after saying yes. [One line: work has locked me into that week and I've lost the fight to move it / money has gotten genuinely tight and I can't do the trip right / a family situation needs me here.] I know this isn't cost free for you, the headcount, the table, all the planning my seat touched, and I'm sorry to be adding a problem during the busiest stretch. I'd like to cover my plate, and I mean that, tell me the number. And I want to celebrate you properly when the dust settles, that part I'm not backing out of.
Three notes on the moving parts. The reason gets one line, and it should be true; a manufactured emergency is a bomb with a long fuse at an event where everyone you know will be comparing notes. If the real reason is soft, the truth still works at low resolution: "some personal things have piled up and I'm not able to do the trip" is honest without being a diary. Offering to cover the plate isn't universal etiquette, and some couples will wave it off, but the offer itself, sincere, with a follow-through if accepted, converts your message from pure withdrawal into shared cost, and it lands especially where money is the reason, because it shows the issue is the whole trip, not their worth to you. And the gift still goes; backing out of attending was never backing out of celebrating.
What not to do, learned from a thousand disasters: don't ghost the RSVP and hope the couple figures it out from your silence. Don't send it through a mutual friend, the couple hears "couldn't even tell us." Don't pad it with so much guilt that the couple has to spend planning week consoling you, we've written about apologies that make the other person do the work. And afterward, don't hide. The instinct after withdrawal is to go quiet out of shame, skip the shower, mute the chat, and the quiet does more damage than the withdrawal, because it turns one canceled seat into a receding friend. Stay loud. Comment on the photos. Book the celebration dinner. The seat was refundable. You aren't.
The day you know is the day you send. Everything else is wording, and the wording above is yours to take.