Dear {Name},
I owe you a huge apology. You should have been invited to {event} but {explain oversight}. I am so incredibly sorry, and so is {host/honoree/family member/associate}.
The worst part is, I don't know how I can make this up to you. The event has passed and you weren't there. It's an error that I cannot correct. I hope that you will accept my sincere apology and know that I value you greatly and if I had it to do over, you would definitely be there with us.
Sincerely,
{Name}
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Index of letter of apology templates