T-Rex: I have an idea for the spookiest night of the year! We shall call it...
T-Rex: ...THE DAY BEFORE ALL SAINTS' DAY!

God: NAME COULD USE SOME WORK T-REX
T-Rex: Fine!

T-Rex: Seeing as I am unable OR UNWILLING to move it from the day before the day Western Christians honour EVERY ONE of the hallowed saints of their church, what if we call it... "The Evening Before The Solemnity of All Saints"?
God: AGAIN THAT DOESN'T REALLY SAY SPOOKY TO ME
T-Rex: Well if that's the case, I don't know what will!!

Utahraptor: "The Night Before All Hallows Day"?
T-Rex: "All Hallows' Evening"?

Utahraptor: Oooh, I think we're getting close. What we make more and more of the name optional, turning "All Hallows' Evening" to "(All) Hallow(s') E(v)en(ing)"?
T-Rex: "Hallowe'en"? Hmm. I like it!! And we can go door-to-door getting CANDY!

God: ALSO YOU CAN GO BOBBING FOR APPLES WHICH IS A FUN WAY TO GET YOUR FACE WET WHILE BITING AN APPLE OTHERS HAVE TRIED TO BITE FIRST
T-Rex: I think that part's less important but sure