Halloween is the current name for what used to be called All Hallows’ Eve and even All Saints’ Eve. Halloween is the day before All Saints Day, also known as All Hallows’ Day, which falls on November 1st.
It’s similar to how people party it up and sin on Fat Tuesday, before the holy day of Ash Wednesday and the start of lent, Halloween is like the evil spirits partying it up and behaving badly before the holy day of All Saints Day.
And in case you’re wondering, there is mass for All Saints Day at Blessed Trinity. It starts with a vigil on Halloween with Holy Day Mass on Friday, November 1st.