Brenda Simpson, from Ruskington, Lincolnshire, said all but one of 82 cards sent to locations across the country arrived on Saturday.
She said: "They've been sitting somewhere and nobody knew anything about them until they all arrived on Saturday, except for one."
Royal Mail said it was investigating the matter.
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Mrs Simpson paid £42 in second-class postage to send the cards to  Lancashire, Yorkshire, Somerset, Devon, Cornwall, Hertfordshire, Bedfordshire, Nottinghamshire and London.
She said she received puzzling phone calls from friends asking why she sent Christmas cards at Easter.
She added: "I posted them early December 2015 to avoid disappointment, yet nobody got them.
"It's very, very upsetting.
"That's a lot of cards to go missing. It's all a mystery."
Mrs Simpson said she submitted a complaint to Royal Mail in 2016 and received £20 in compensation, despite never receiving an explanation of what had happened to the cards.
One card, containing a £10 note was yet to turn up, she said.
The pensioner added she would not be sending Christmas cards this year "because they've already received them".
A batch of Christmas cards posted in 2015 have finally arrived at their destinations more than 16 months late.