deathday
The day of a person's death, especially when viewed as being appointed by fate.
The placing of the dragon is inevitable: a man can but die upon his death-day.
The anniversary of a death, the date upon which someone died in a bygone year.
"Is it her deathday?" I asked. "Nope," he said.
Where it comes from
From Middle English deth-day, deth day, dethe-day, deþ day, from Old English dēaþdæġ (“day of one's death, deathday”), equivalent to death + day. Compare birthday. Compare also German Low German Doodsdag (“deathday”), German Todestag (“deathday”), Danish dødsdag (“deathday”), Swedish dödsdag (“deathday”).
Adapted from the Wiktionary entry deathday (CC BY-SA 4.0).