Explanation:

- The idiom "Once in a blue moon" refers to events or occurrences that happen very rarely.

- Option 1: Something that happens very rarely – This option is correct. The phrase describes events that do not occur frequently.

- Option 2: To get a rare acknowledgment from someone you look up to – This option is about receiving rare recognition or praise, which is specific to interpersonal relationships and not applicable here.

- Option 3: When occasionally your luck helps you in dire situations – This implies sporadic good fortune, but is not the accurate meaning of the idiom.

- Option 4: To find hope in an unexpected situation – This option describes finding unexpected hope, but this is not aligned with the idiom's meaning.

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