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Happy New Year.
Literal
Open-come [honorific-まして] honorable-congratulations.
あけましておめでとう is the standard New Year's greeting, literally 'the year has opened [to the new], (I offer) honorable congratulations.' The verb is 明ける ('to open, to end [a period], to dawn'), here referring to the old year 'opening out' into the new. A greeting tied to a specific time window — only used during shogatsu (正月), typically January 1-7.