The festivity is primarily a new Celtic fire festival, midsummer [1] representing the midst of summer, along with the shortening of the days on their gradual march to winter. Midsummer is usually celebrated on either your 23rd or perhaps 24th associated with June, even though the longest day time actually comes on the 21st of June. The importance of the day to our ancestors can be tracked back thousands of decades, and many gemstone circles and other ancient monuments are generally aligned towards the sunrise in Midsummer's Day. One of the most famous place is that at Stonehenge, where the sunlight rises on the heel rock, framed from the giant trilithons about Midsummer morning.
Links:
[1] http://summersoltice.co.uk