Litha is also known as Midsummer or the Summer Solstice. It is a celebration of the longest day of the year, when we get the most daylight hours, and as such it is a Fire Festival.
At Litha, the Goddess is pregnant, full of potential and life. The God is also at the height of his virility. Bonfires are lit to symbolise and honor the fullness of the sun, and the embers are scattered on the fields (also full of potential and ready to produce) to ensure a good harvest.
As a sabbat, Litha also represents a turning of the wheel, the movement of the seasons. As we celebrate the light and fullness of the sun we must also acknowledge that the longest day represents the start of the move back towards darkness. This is part of the balance of the duality of nature.
Oak, which is traditionally used on the Midsummer bonfires, is used because the Oak King rules over the waxing first half of the year, but now he concedes to his brother the Holy King. The celtic name for Oak is duir, meaning doorway, and here again we get the sense that even at the height of our celebrations of summer we cross the threshold into the next part of the year.
Pagans celebrate the festival using herbs, flowers, and honey (the June moon is known as the Honey Moon)