Aging George Stransom holds sacred the memory of the great love of his life, Mary Antrim, who died before they could be married. One day Stransom happens to read of the death of Acton Hague, a former friend who had done him a terrible harm. Stransom starts to dwell on the many friends and acquaintances he is now losing to death. He begins to light candles at a side altar in a Catholic church, one for each of his Dead, except Hague.
Later he notices a woman who regularly appears at the church and sits before his altar. He intuitively understands that she too honours her Dead, and they very gradually become friends. However Stransom later discovers that her Dead number only one: Acton Hague. Hague had wronged her too, but she has forgiven him. When his friend realises Stransom's feelings about Hague, she declares that she can no longer honour Hague at Stransom's altar. Stransom cannot bring himself to resolve the issue by forgiving Hague and adding a candle for him. This disagreement drives the two friends apart. Stransom's friend ceases visiting the altar, and Stransom himself can find no peace there.
Months later, Stransom, now dying, visits his altar one last time. Collapsing before the altar, he has a vision of Mary Antrim, and it seems that Mary Antrim is asking him to forgive Hague: "[H]e felt his buried face grow hot as with some communicated knowledge that had the force of a reproach. It suddenly made him contrast that very rapture with the bliss he had refused to another. This breath of the passion immortal was all that other had asked; the descent of Mary Antrim opened his spirit with a great compunctious throb for the descent of Acton Hague."
He turns and sees his friend, who has finally become reconciled to him, having decided to visit the altar to honour not her own Dead but Stransom's. Stransom, dying, tries to tell her that he is ready to add a candle for Hague, but is able only to say "One more, just one more". The story ends with his face showing "the whiteness of death." Thus Stransom's last words are rendered ambiguous.

Question: When did Mary Antrim die?
Answer: The day before her marriage to Stransom.
Alternative answer: Before they could be married