Different cultures celebrate Midsummer on different days, as the actual summer solstice changes each year (this year it's on June 21.) From this article by Kala Kumar:
"Traditionally, Midsummer is the point when the veil between the fairy world and the mortal world lifts, and fairies and human are believed to mingle more during this period than at any other time of the year.
"Flowers, ferns, and herbs gathered on the solstice and blessed by the fairies, were believed to have special magical properties, including the ability to predict the future."
If you live in a place believed to be haunted by the faerie folk, keep your eyes peeled!
And since I'm a flutist, I have to share this too: