On the eve of the first of November, not only is Halloween celebrated, but also "Samhain", the Irish-Celtic festival of darkness and the encounter between the living and the dead, when the gates of the Otherworld open.
Samhain forms the dark pole of the Celtic year, while Beltane (on the eve of the first of May) marks the beginning of summer and the start of the Celtic year in the Irish calendar.
The connection with Halloween is controversial: the fact is that Halloween is also celebrated on the evening before All Saints' Day (November 1st) - i.e. on "All Hallows' Eve", which has developed into "Halloween" over time. All Saints' Day was officially set on the first of November in the 9th century by the then Pope Gregory IV to commemorate the saints. This is followed one day later by "All Souls' Day", when intercession is made for the deceased.