The act or practice of celebrating Christmas; festive activities associated with Christmas.
From Christmas (the holiday) plus the gerund suffix -ing. This formations allows Christmas to be used as a verb ('to Christmas') and creates a noun for the action of celebrating.
The word 'christmasing' is delightfully awkward and old-fashioned—it's what Victorians called all the shopping, decorating, and caroling we'd now just say 'Christmas preparations' or 'holiday stuff.'
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