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A day trip might be possible, but not much fun. The train to Bayeux takes two hours and a quarter, so round trip that's 4 hours and a half. From Bayeux to Arromanches or Longues-sur-Mer you can take buses, round trip of about half an hour. So that's 5 hours, plus waiting time, out of a day, just to go and come back.
That's the irony of the train system: it takes less time to do Paris>Brussels than Paris>Bayeux...