unwedding ceremony
n. A formal ceremony held to celebrate a couple's divorce and to acknowledge their married life.
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Now that divorce is an established cultural tradition, and no longer stigmatised as a shameful moral failing, increasing numbers of incompatible Americans are choosing to solemnise the break-up of their marriages with an "unwedding ceremony" — often in church with a reception afterwards — which acknowledges their shared life and marks their amicable separation as a couple.
—Dermot Purgavie, “More and more divorcing couples are opting to end their union with a formal ceremony,” Sunday Express, June 17, 2001
1980 (earliest)
George Zweibel of Washington, D.C., son of the late Albert Zweibel, was unmarried last evening at a Silver Spring warehouse in a simple ceremony officiated by William Berinsky, a bass guitarist.

"Will you, George, continue to keep this woman as your wedded wife, love and comfort her, in sickness and in health, etc., etc.?" said Berinsky.

"No way," said George.

The guests cheered.

This was George Zweibel's official "unwedding," thrown by two cohorts to celebrate his messy divorce granted July 1. "This is the happiest day of my life," said Zweibel.
—Elisabeth Bumiller, “Tying the Not,” The Washington Post, July 19, 1980
To divorce lecturer and author Rabbi Earl Grollman of Temple Beth El in Belmont, Mass., divorce can be even more traumatic then death. 'The big difference is, death has closure, it's over,' says Grollman, who performs divorce ceremonies for families. 'With divorce, it's never over.'
—Linda Bird Franke, “The Children of Divorce,” Newsweek, February 11, 1980