Definition of “not be sb's day” - English Dictionary

british dictionary

“not be sb's day” in British English

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not be sb's day

If it is not your day, you are having a difficult or unpleasant day:

This really isn't my day - my wallet was stolen and now I've lost my car keys.

(Definition of “not be sb's day” from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)