Michelmas isn't pagan, it is the mass of St Michael and All Angels, September 29th. The only festival that would tie in with would be the various harvest festivals and the autumnal equinox around the 21st-23rd. There would be plenty of other food around then.

The boars head ceremony celebrated on the Saturday before Christmas commemorates a student of Queen's college, Oxford, who was charged by a boar whilst walking in the nearby forest of Shotover. He was studying Aristotle, when the boar rushed him. The student thrust his copy of Aristotle down the beast's throat, shouting 'Graecum Est!!' (it's in Greek) and escaped.

Boar was traditionally eaten at Yule in Scandanavia, in honor of the sun-boar, and the Celts believed it was an agent of evil, a belief encouraged by the early church. Psalm 80 mentions a vine (the Jewish nation) being brought from Egypt and planted (in Israel) and then uprooted by the wild boar (persecution of the children of God) so the boar stayed as a baddie, and the boars head eating continued.

LTS, quoting Frank Muir.