The Roman Britain website also contains the following related quote :-

"Though not confirmed by the Coligny inscriptions, we also know that in accordance with general Celtic custom, which was itself adopted from the ancient Greek observance, each day was reckoned to last from sunset to sunset; not midnight to midnight as our modern Roman calendar dictates."

The idea of starting a day at sunset exists elsewhere such as in Judaism and Islam. It strikes me as being a more obvious method of picking a point to begin and end the day, along with the possibility of sunrise, as they would seem to be observable events. How do you work out that it is midnight ? The effect of starting the day at sunset as the Celts apparently did would be that once a Roman calendar is introduced the dating of celebration days would leave them spread over 2 of the new days. This may go some way to explaining the extensive use of 'eves' to start the celebration at sundown and continuing into the 'next' Roman day. In this way when Samhain is placed on the new calendar it becomes Hallowe'en (All Hallows eve) and then All Hallows itself.