Thomas Keightley – The World Guide to Gnomes, Fairies, Elves and other Little people

The elves are extremely fond of dancing in the meadows, where they form those circles of a livelier green which from them are called Elf Dance – Elfdans.  When the country people see in the morning stripes along the dewy grass in the woods and meadows, they say the elves have been dancing there. 

If any one should at midnight get within their circle, they become visible to him, and they may then illude him.  It is not every one can see elves, and one person may see them dancing while another perceives nothing.  Sunday children, as they are called – that is those born on a Sunday – are remarkable for possessing this property of seeing elves and similar beings.  The elves, however, have the power to bestow this gift on whomsoever they please.... the sound of their voice is said to be sweet and soft like the air