Martin Brennan says that the mound of the hostages at Tara is aligned to the cross quater days, November 8th being one of these (like Cairn L at Loughcrew) the drawing in his book shows a rectangle beam which hits the backstone as soon as the sun climbs above the horizon.

Samhain has always been marked at Tara, I even remember teachers talking about his when I was in Primary school though I am not sure if there is an explanation why this date was chosen. It could well be that its the end of the harvest or shifted over the years from the cross quater day on the 8th November, maybe when it was christianised into 'all saints day'.

I'd love to hear any input on this, before I knew anything about the winter/summer solstice or the equinoxes as a kid Halloween/Samhain has always been a big deal so its importance has been strong enough to survive the millenia, must be for a good reason.