Sacrifice to the elves. There are three attestations in Old Norse literature to this sacrifice, the earliest of them, that of the skald Sigvarr Þórðarson in 1018. It occurs again in Kormaks saga 22 though the ritual is somewhat different. As Simek says, the internal chronology of the saga puts the event at the mid-10th century though the saga itself was written in the first half of the 13th. The third account comes in the Ynglinga saga 48, 49 in the account of the reign of King Oláfr Guðroðarson. Simek's conclusion is that Ëven if we do not know anything definite about the álfablót, it is nonetheless possible that the elves (as opposed to the dwarfs) had a cult devoted to them." This lack of specific information regarding a ritual has not stopped modern day Heathens from adopting it and giving it substance. Unfortunately, unlike Mediterranean Pagans (Greeks, Romans, etc) Norse Pagans did not leave a written record of their beliefs, customs and rituals and Christianity was very thorough in stamping out the ethnic religion of Scandinavia. As cultural genocide goes, they were unfortunately quite diligent.
Rudolf Simek, Dictionary of Northern Mythology (Cambridge: D.S. Brewer, 1993), 7-8.