Ulfrikr inn Hrafn
Samhain is the Celtic name for the Midsummer rite.
Actually, it's their fall rite, opposite Bealtaine. The Celtic summer rite is (and I will butcher this) Lughnassadh, and their Winter/Early Spring one is Imbolc.
Samhain is, in Celtic trad, one of the nights when the veil between the worlds is thinest and the dead come out to play. I think it also has ties to the Wild Hunt of Celtic lore, which is similar to the Wild Hunt of Norse lore. I've always mentally categorized it as similar to the 12 days of Yule, when only a fool would be abroad at night due to the Wild Hunt (and that whole long nights/cold/freeze no matter what, thing).
The quality of it is different, though, if that makes any sense. Norse rites I know of have a strong emphasis on life, and Samhain has always struck me as very much about death. This is appropriate for a people as stuck on liminal spaces as the Celts are, though. For the one Norse-themed Samhain I once participated in (and Gods do I wish they'd consulted me on the guided meditation; they got the directions wrong, and I was completely thrown. I ended up spending the entire time in Yggdrasil's roots recovering.) we honored Hel and (since they asked me - they were going to do Tyr at first, I think!
gonk ) Baldr. So far as I know, this was a historically very odd rite... though it wasn't bad as things go.
Honestly, the longer I've been Asatru the less fitting in with the general pagan population has mattered to me. I get along quite well with the local circle, they understand my boundaries (I will HP if the gods are the Aesir and Vanir, but no other time, and I'm iffy on calling quarters), and I understand their rites. It works, and in a lot of ways that's what matters to me.