unwedding ceremony
(UN.wed.ing sayr.uh.moh.nee) n. A formal ceremony held to celebrate a couple’s divorce and to acknowledge their married life. Also: unwedding.

Example Citation:
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

Earliest Citation:
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

Another name for an unwedding ceremony is a divorce ceremony, a phrase that goes back over 20 years:

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

If you see the end of your marriage as more of a death, then perhaps a marriage wake (2004) is more your style:

It was a lovely ceremony. The bride wore black.

And if her heavy veil, bowed head and crumpled tissue said "grieving widow," it was only fitting. For this was not a wedding, but an unwedding — a "marriage wake" to mark the death by divorce of Amanda Schultz's 11-year union with her dearly departed.
—Cecilia Goodnow, "Requiem for a marriage," The Seattle Post-Intelligencer, February 3, 2004

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