Midsummer Night's Eve is supposed to be a good time to commune with field and forest sprites and faeries. The faeries abound at this time and it is customary to leave offerings - such as food or herbs - for them in the evening.
Midsummer's Eve at midnight, Midsummer's Day at dawn and Midsummer noon are prime times to collect plants sacred to the sun or special to the fey.
Midsummer is a fey time, both by tradition and observation. The scent of the air is thick, green and juicy; it's lost its spring astringency and is simply lush. The whole world is stretching its limbs and frolicking. The fey are big on that. Especially for charms of love, gardening and magickal abilities, the fey are a great help in herb collecting. In exchange, they like gifts of milk and honey, cookies, sweet liqueurs, or any sweet food, drink or liquor. They also like baubles, particularly pretty or shiny. Or cold hard cash -- but in coin, not paper, and it's best if shiny.
To stay in good with the fey and the herbs you collect from, leave enough of the plant or other plants of the type that the herb survives in the spot collected from. Remember too to always ask the plant before taking a cutting, and to wait for an answer.
Fey charms: To see the fey, pick flowers from a patch of wild thyme where the little folk live and place the flowers on your eyes. A four-leafed clover not only grants you a wish but also, carried in your pocket or a charm, gives you the power to see fairies dancing in rings. A good place to look is by oaks, said in Germany to be a favourite place for fey dances. To penetrate fey glamour, make and wear an ointment including four leaved clovers.
St. John's wort, also known as ragwort, has a strong connection to the fey and transportation. You might add it to charms to travel quickly. The Irish call the plant the fairy's horse, and the fey are said to ride it through the air. But beware: The Manx say if you step on a ragwort plant on Midsummer's Eve after sunset, a fairy horse springs out of the earth and carries you off till sunrise, leaving you wherever you happen to be when the sun comes up.
(various authors - unknown to me)