Correct spelling


Correct spelling, explanation: the adjective mystic, which is the core word, in this case, points to the spelling with y. It comes from Greek mystikos and by adding the noun suffix -terion we received mysterion. Therefore mystery is the correct spelling, while such spellings as mistery are mistakes.

Definition of mystery:
noun, something unknown, unexplained; also: a genre of a novel or short story involving crime
The origins of stone statues on Easter Island remain a mystery.
This mystery by Agatha Christie is my favourite crime story.

Incorrect spelling


Incorrect spelling, explanation: mistery is a misspelling, probably mistaken due to the word misery, which is similar but spelled with i. The correct form is mystery and it comes from Greek mysterion. Thus the only correct spelling is mystery, with y after m.