Fairy
Over hill, over dale,
Thorough bush, thorough brier,
Over park, over pale,
Thorough flood, thorough fire,
I do wander everywhere,
Swifter than the moon's sphere;
And I serve the fairy queen,
To dew her orbs upon the green.
The cowslips tall her pensioners be:
In their gold coats spots you see;
Those be rubies, fairy favours,
In those freckles live their savours:
I must go seek some dewdrops here
And hang a pearl in every cowslip's ear.
Farewell, thou lob of spirits; I'll be gone:
Our queen and all our elves come here anon [ref. 6, Act 2, scene 1]

June 21st is known as Midsummers Night Eve, the magical night that Shakespeare wrote his play about; the enchanted evening when fairies wander, and mortal eyes can glimpse them.

The origination of this legend is in Engand's myths.  This was the evening when Fairies are given the gift of human speech.  If you find one, it may be in the middle of a bunch of flowers or mushrooms that have grown because they are out galavanting.  If you eat one of the mushrooms, you may even see a lot of them [ref. 4].  To atract them, leave cookies and plant herbs [ref. 1].

Asherah, a wiccan practitioner, lists a number of spells that can be done on this eve.  Most involve planting things [and she throws in a wicked love spell in there as well], but here is a nice luck charm from the old times,

"On Midsummer Eve at midnight, the fern blooms with a golden flower. If you pluck this flower, it will lead you to golden treasure. In Russia, the flower must be thrown in the air, and it will land on buried treasure. The Bohemians believe that if you pluck the flower and on the same Midsummer Night climb a mountain with the blossom in hand, you will find gold or have it revealed to you in a vision. Bohemians also sprinkle fern seed in their savings to keep them from decreasing." [ref. 2]

Fairekeeper.net has a nice little entry about the history of this holiday, and the celebration they have, and incorporating the Christian element, St. John's [the Baptist's] Eve.  They throw a party and and pretend to be faerie's.  [ref. 3]

And on June 20 - June 24th the fairies have been witness in force [ref. 5].  A White Lady, one of the most powerful of the Sidhe is seen in Hapmton Court, Surrey.  King Arthur's Table is seen in Cornwall at the Bossiney Mound, and at the Isle of Man, Fairies are seen crossing the bridge.  It may be a bit dangereous to be in their path, unless you have a bit of Iron in your pocket.

ref. 1 http://www.chamomiletimes.com/articles/fairiesingarden.htm

ref. 2 http://www.widdershins.org/vol1iss2/4.htm

ref. 3 http://www.faeriekeeper.net/freedom.htm

ref. 4 http://www.thegooddrugsguide.com/mushrooms/faq.htm

ref. 5 http://www.paranormaldatabase.com/calendar/Pages/jun.php

ref. 6 http://www.online-literature.com/shakespeare/midsummer/