I can't really think about Halloween, or Samhain, if you prefer, without thinking of the ballad of "Tam Lin," especially this part:

"And pleasant is the fairy land,
But, an eerie tale to tell,
Ay at the end of seven years
We pay a tiend to hell;
I am sae fair and fu o flesh,
I’m feard it be mysel.

"But the night is Halloween, lady,
The morn is Hallowday;
Then win me, win me, an ye will,
For weel I wat ye may.

"Just at the mirk and midnight hour
The fairy folk will ride,
And they that wad their true-love win,

"Tam Lin" Child Ballad 39A.24

"Tam" Lin is one of the Child Ballads; a collection of early English and Scottish popular ballads collected by Francis James Child. Most of the Child ballads are from the sixteenth century. Some few are older. You can find a decent list of the Child ballads by number here. A fair number of the ballads are much older than the earliest printed sources; "Tam Lin" is one of those. It's also one of the best known; there are lots of covers by folk rock bands, as well as more traditional singers. The basic story line of "Tam Lin" tells of a mortal man kidnapped or "taken" by the queen of the fairies when he falls off her horse. He is destined to be sacrificed on Halloween, as a teind or tithe to Hell, unless his mortal and pregnant lover Janet rescues him.

You can find the entire text of the ballad in multiple versions here. One of the reason's I find the story of Tam Lin so very compelling is that it's very much tied to the idea of seasons, and to the Celtic idea that at Halloween the Otherworld is closer to this world, and thus allows more ready passage between the two. Halloween is a liminal time, in that sense, as is Samhain. When Janet rescues her lover, it is at midnight, a time between times, at a crossroads, a place between places.

A fair number of fantasy and SF writers have used all or parts of the ballad in their books. Pamela Dean's Tam Lin (that's the cover up there at the top, with a link to Dean's Web site), and Elizabeth Marie Pope's YA The Perilous Gard are two of my favorites. Other writers, principally Patricia McKillip in Winter Rose and Elizabeth Bear in Blood and Iron and Whiskey and Water use the ballad in interesting and compelling ways.

Probably the best known cover of Tam Line is this 1969 performance by British folk rock band Fairport Convention, from their lovely Liege and Leaf album.