alt Hacker News> https://www.historyhit.com/what-are-the-origins-of-halloween...
All Saints Day, which “All Hallows Eve" is the vigil celebration of, came not from the US or the Celts:
> […] so the Orthodox tradition celebrated All Saints on the Sunday after Pentecost (as it still does), while Syrian tradition held it on the Friday after Easter. On 13th May 609 AD (or perhaps 610 AD) Pope Boniface IV consecrated the Pantheon in Rome as a Catholic church dedicated to all saints and ordered an annual celebration of the saints in that church on this date, which is held to this day. At some point in his pontificate (731-41 AD), Pope Gregory III dedicated a chapel in St Peters to all saints and martyrs and some accounts say this was on November 1, making this the potential origin for the western date for All Saints Day.
* https://historyforatheists.com/2021/10/is-halloween-pagan/
This date then became popular under the rule of Charlemange:
> This urging suggests that All Saints Day was perhaps celebrated on other dates and Alcuin, by this stage back at the court of the Frankish ruler Charlemagne at Aachen, preferred the tradition he knew from England. Or it could be that he is urging the importance of the feast rather than the date of the celebration per se. What we do know is the November 1 date caught on in Frankia, with Pope Gregory IV promulgating it as the date for All Saints Day for both East and West Frankia, and this was reinforced by an edict by Louis the Pious in 835 AD as recorded in the entry for that year in the Chronicon of Sigebert of Gembloux. With the date established across the Frankish Empire, it became more widely adopted and over the next two centuries became standard Catholic liturgical practice across Europe.
* Ibid
But it was not a universal date yet, as the Irish/Celts celebrated the feat on a completely different day:
> The Félire Óengusso or “Martyrology of Óengus” is another martyrology, attributed to Saint Óengus of Tallaght. It seems to date to the ninth century and is based on earlier English martyrologies (like that of Bede), but with significant local Irish additions. It mentions a feast of All Saints in its listing for April 20:
* Ibid
The History Hit article mentions Pope Gregory VII (1073–85), but November 1 was already around before he got there. He mainly told the folks celebrating in April/May to stop and to celebrate in November.
* https://en.wikipedia.org/wiki/All_Saints%27_Day § History
The octave wasn't added until much later, under Pope Sixtus IV (1471–84), and so any kind of festivities on October 31 (vigil) were even later.