If you're canceling a wedding because of coronavirus, we've got a more specific post about that:
How to notify guests about a canceled wedding after a breakup
Sadly, we have decided to call off the wedding. We sent out the Save The Dates a while ago, and we're going to each notify our own “sides” of the guest list.
I can call my friends, but what do I do to notify the distant relatives and parents' friends? I know I need to send something soon because the wedding date is only 3 months away. In this state of mind, I can't think of what to say.
First: I'm so very sorry. This is one of those situations where simplicity goes a long way — you don't need to get into details with extended family. A phone call may work better than a mailed card, which could get lost in the shuffle.
If you're close with your parents and can ask them to call the distant relatives and family friends, that's the best bet. It's likely that they've got the contact information anyway, and they've got the relationships with these people and can field the possibly-intrusive questions that guests may have.
If you don't feel comfortable asking your parents, you can make the calls yourself. Be gracious but don't get into details:
I just wanted to let you know that we've decided not to move forward with the wedding. I hope I've caught you before you've made any travel plans.
If they ask why the wedding's been called off, simply say, “It was a decision that we put a lot of thought into. I don't really want to talk about it, I'm sorry.”
No need to go any further.
Canceling a wedding is a deeply painful process — but less painful than a divorce a few years down the line.
Best of luck, and lots of love.