The story
From a Scots shortening of Allhalloweven, from Allhallowmas (the obsolete earlier name of All Saints' Day) + even (an archaic or poetic variant of eve; see also een). For the American pronunciation with /ɑ/, compare the Southern English traditional dialect pronunciations fallow /ˈfɒ.lə/ and tallow /ˈtɒ.lə/.
Etymology from Wiktionary, CC BY-SA 4.0.