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Look up m-day in Wiktionary, the free dictionary.
M-Day may refer to:
- M-Day (book) (Russian: День „М”), a history book by Viktor Suvorov
- M-Day, in the Marvel Comics Decimation storyline
- M-Day, a military designation of days and hours for the day on which mobilization commences
- Mobilization Day, the day Canadian police would arrest suspected communists under the secret PROFUNC program