Hallow is derived from Old English (= OE) hāliȝ meaning 'holy'. One of its inflected forms is hālȝa. This developed into halȝa (shortening before Cs). The <ȝ> was pronounced [ɣ] before back vowels in OE times; in unstressed syllables this sound became an u-like vowel, and later on probably due to the preceding [l] a diphthong containing an [u] as its second element - written <ow> - came into existence. According to OED the word hallow was little used after 1500 (see date chart):
Hallowe'en seems to have Celtic origins according to Nicholas Rogers (2002), Halloween, OUP . In the medieval Irish calendar the festival of Samhain (pronounced /saʊn, ˈsɑːwɪn, ˈsaʊən/) was celebrated to mark stock-taking, the yearly harvest, to greet the imminent winter etc. Rogers continues:
It was also a period of supernatural intensity, when the forces of darkness and decay were said to be abroad, spilling out from the sidh, the ancient mounds or barrows of the countryside. To ward off these spirits, the Irish built huge, symbolically regenerative bonfires and invoked the help of the gods through animal and perhaps even human sacrifice. (12)So it was basically a non-Christian, heathenish festival. And what is it today? Ask me not!