And hails, to pensive damsels dear,
This eve, though direst of the year...
Oft on the shrub she casts her eye,
That spoke her true-love's secret sigh;
Or else, alas! too plainly told
Her true-love's faithless heart was cold.
'Cottage Girl', a poem from Midsummer eve, 1786
According to British Folk Customs, in what is claimed to be a continuation of ancient pagan ceremonies, every June 23 a series of fires are lit across Cornwall, the lie stretching from Lands End to the northern border with Devon, or the English border as some Cornish people look at it.
These days the midsummer bonfires are fed with greenery, one bunch of good, medicinal plants, another a bad bunch of noxious weeds and bitter herbs, both of them bound with ribbons of symbolic colours. But many centuries ago, according to some sources, there were animal and even human sacrifices made the offerings by being plunged into the flames. When Christianity came to Britain the church embraced the custom, while ensuring it was associated with a Christian saint as it took place on St John’s Day eve.
The tradition faded in Victorian times, but was revived when the Old Cornwall movement started in the 1920s. Now fires are lit in Redruth , St Just , St Columb , St Breock, St Agnes , Madron and St Ives , still with the Old Cornwall movement to the fore, and with the ceremony conducted in the Cornish language.
The following quotations are posted from the book "Traditions and Hearthside Stories of West Cornwall - Vol. 2" - by William Bottrell -  - (Obtain this full work and many more backups by clicking here!)
Our bonfires, torches, and tar-barrels, with the peculiar hand-in-hand dance around the blazing piles, remind us of ancient times when similar customs were regarded as sacred rites by our forefathers; and it would seem a s if some vestiges of these time-honoured religious notions were still connected with Midsummer bonfires in the minds of old-fashioned people, living in remote and primitive districts, where they still believe that dancing in a ring over the embers, around a bonfire, or leaping (singly) through its flames, is calculated to insure good luck to the performers and to serve as a protection from witchcraft and other malign influences during the ensuing year.
We are sorry to find that another pleasing Midsummer's observance, which also appears to be ancient, has almost died out. Yet within the memory of many, who would not like to be called old or even aged, on a Midsummer's eve long before sunset, groups of girls—both gentle and simple—of from ten to twenty years of age, neatly dressed and decked with garlands, wreaths, or chaplets of flowers, would be seen dancing in the streets.
One favourite mode of adornment was to sew, or pin, on the skirt of a white dress, rows of laurel-leaves, often spangled with gold leaf. Before Midsummer small wooden hoops were in great demand to be wreathed with green boughs and flowed for garlands, to be worn over one shoulder and under the opposite arm. Towards sunset groups of graceful damsels, joined by their brothers, friends, or lovers, would be seen "threading-the-needle," playing at "kiss-in-the-ring," or simply dancing along every here and there from Chyandour to Alverton, from the Quay to Caunsehead, as the upper part of the town used then to be called, perhaps with more propriety than Causewayhead.