The Summer Solstice is as astronomical event which happens around the 20-22nd of June in the Northern hemisphere. It varies a little from year to year because of the way the Earth wobbles on its axis. Some groups stick to the astronomical date and their celebration will also vary to follow this.
The Druids always use the astronomical date to time their celebrations at Stonehenge.
This will mean that midsummer's eve will be the night before the Solstice.
However in the UK Midsummer's Day has been celebrated on the 24th of June for centuries, and this is also the case in other countries too. This means that the evening of the 23rd of June is Midsummer's Eve, and the night that traditional folklore relates to.
Although if you take the UK calendar reform into account*, when William Shakespeare wrote his play 'A Midsummer Night's Dream', then Midsummer's eve would fall on the equivalent of the 5th of July, and there are traditional groups who adhere to the Old Calendar.
This year the next Full Moon is on the 28th of June, and there are yet other groups who use the Full Moon nearest to celebrate their festivals.
I know some people do find this confusing. I call it 'being given a choice'. It also means that you can pick one date for your private Witchy celebration, and another one for a more inclusive ask-the-neighbours-round party.
It is said the on midsummer's eve the doors to the Fairy Mounds open, and the Fairy Queen and her court ride the land. In Witchdom we say that around the times of the festivals the doors between the world grow thin. In other words it is easier at these times for spirits to cross from their worlds into ours and vice versa. This is not a single day happening, it is a time of around a fortnight or so during which it is easier to access these alternate worlds.
Around the Solstice the magical fern flower is said to appear and glow in the darkened woodlands. If you can find it and catch the fern seed in a pewter dish, it can grant you the power of invisibility.
Litha is a fire festival, and is celebrated by having an outdoor bonfire. Barbecues can be the modern equivalent of this, so why not have a midsummer barbecue party!
* In 1752 the English parliament decreed that we should move from the old Julian calendar to the new Gregorian calendar, simply by missing out eleven days in the year, which did cause rioting in the streets by people thinking they had somehow been robbed of eleven days..