So, which Chaos God best embodies Samhain ?

Nurgle , representing the harvest's 'circle of life'? The start of the season of decay?

Tzeentch , for the change in seasons? Changing personality by wearing a mask?

Khorne , for the blood sacrifices that ensure a good harvest? Turning the strength of the earth into the strength to destroy enemies?

Slaanesh , presiding over the celebration at the end of the harvest? For the sensual pleasures of the fruits of the field?

At first impulse, Nurgle for sure. Though you can make a serious case for Tzeentch,

I believe the third Eisenhorn novel Hereticus made an off-hand mention of the pre-Imperial Terran 'Eve of Hallowing' and present-day (i.e. M41) offshoot practices, linking them to Khorne worship and blood sacrifice.