http://en.wikipedia.org/wiki/Tam_Lin
"Every seven years the fairies paid a teind, a tithe to Hell, of one of their people, and Tam feared he would become that tithe on that night..."
http://www.tam-lin.org/interp.html
"Every Seven Years: Seven years crops up as a specific length of time in several other settings. In ballads such as Botany Bay as the length of time a man can be sentenced to transportation for a crime. It occurs in many cultures as term of punishment or length of contracts. It's the length of time that the Queen taked True Thomas to Faerieland in some versions of Thomas the Rhymer. In some Celtic cultures it was the length of time that an apprentice would serve under a master while learning a trade. Under English law, seven years' absence was long enough to declare a person legally dead. Given all of these seven-year cycles, it's possible that Tam Lin could only go free at the end of a seven year cycle, not only just on halloween night or just when someone was willing to capture him."
