Halloween is "Hallow's Evening". ie the Evening of All Hallow's Day, aka All Saint's Day, and directly related to Dia de los Muertos. But it's the calendar day before, right? October 31st? Sort of. In Catholicism, the liturgical day begins and ends with prayers at sunset. The day had ended and a new day had begun, usually between 4pm and 6pm. So the order of the 24 hour day goes: evening, night, morning, afternoon. Therefore 6pm on the 1st of November would be too late to celebrate All Saint's Day, so it was actually celebrated the calendar day before.

The same is true of Christmas Eve. Why do we have the "Evening" of Christmas the day before? Why do many cultures begin full celebration the night before? Precisely because as a Christian event on a Christian calendar, that is when the day actually begins.

As a former Catholic it's important I acknowledge all of this overlays the local traditions syncretised into these far more secular 'holidays' today. It wouldn't be important for Halloween to be celebrated in the "e'en" if Oiche Samhain wasn't a night time thing.