i might be thinking of may day... yeah the notion was that infidelity was acceptable for the duration of the festival.

> May Day. Ancient Rites that survived. Celts had several festivities such as Beltane which was the last and largest of the spring festivals, falling on April 30th or May 1st (its name come from “Bealtaine” in old Gaelic, which means “fire of Bel” the Celtic god of light); the other festivals were Ostara (now Easter), the Summer Solstice and Samhain.

Beltane, as a custom survived in various parts of Europe, such as France, Germany and England. It included setting up a village May-pole. It involved dancing and great sexual frolics around this gigantic phallic symbol which represented gods phallus in Mother Earth. After dancing around the Maypole celebrants would retire to the open fields where they would have sex with anyone and everyone in the plowed fields in order to insure the fertility of the land and prosperous yield of crops3.

Napier 6 concluded that the English May feasts are a survival of Roman Floralia, introduced by Rome into Britain and imposed on the original Celt population, which kept it and nutured it after the fall of Rome. He also says that during the middle ages, they "were not free from some of the indecencies of the Floralia". Judging by the dates, May Day also coincides with Faunalia, so a mixed origin of different fertility rites can be assigned to this Celtic celebration.

For these reasons of sexual license, the Christian Church opposed May festivals and the sexual freedom it promoted: Phillip Stubes, an English Puritan writer said (Anatomie of Abuses - 1583):
"What clipping, what culling, what kissing and bussing, what smooching and slobbering one of another, what filthy groping and unclean handling is not practiced in the dances."

Regarding free sex in the fields, says Stubbes:
"Against May, Whitsonday, or other time, all the yung men and maides, olde men and wives, run gadding over night to the woods, groves, hils, and mountains, where they spend all the night in plesant pastimes; ... and in the morning they return... there is a great Lord present amongst them, as superintendent and Lord over their pastimes and sportes, namely, Sathan, prince of hel. ... I have heard it credibly reported (and that viva voce) by men of great gravitie and reputation, that of fortie, threescore, or a hundred maides going to the wood over night, there have scaresly the third part of them returned home againe undefiled."4. Another Puritan wrote that men "doe use commonly to runne into woodes in the night time, amongst maidens, to set bowes, in so muche, as I have hearde of tenne maidens whiche went to set May, and nine of them came home with childe" 7.

But all good things come to an end, and the sixteenth century saw the end of May freedom. The Puritans in England made the Maypoles illegal in 1644. They also attempted to suppress the greenwood marriages of young men and women who spent the entire night in the forest. The practice continued for some time, but eventually died out.

http://www.eioba.com/a70752/orgies_a_brief_history