The Celtic Tree Month of Oak starts on June 10, 1999 and ends July 8, 1999. It's symbol is the White Horse or the Golden Wheel. Its ruling planet is Jupiter/Jovyn. It is associated with the God Dagda.
In the Druid philosophy the Oak Tree represents the turning point of the year when the solar god, Taliesin entered into the circle of Gwynvynd to recieve divine inspiration (Awen). Druids would carve a circle divided into four parts (the symbol of earth) into an Oak Tree for protection from lightning.
The Oak's letter word Duir is descended from the Sanscrit word dwr meaning door. Oak doors were the strongest against evil, then we developed steel. The Sun king, Taliesin, would symbolically enter the portal from the earth realm into the underworld realm of Pwyll which contained the "fiery abyss of Annwn". He would continue this journey until the Winter Solstice when he confronts his dark twin, Avagddu. They would meet in the middle of the "ford of darkness, where he would defeat him and regain his solar divinity. Annwn was considered an astral dimension of Earth.
There are three circles through which the soul travels in Druid lore. The first is the Circle of Abred where mankind dwells and good and evil exist in equal measure, where all things seek balance. Mankind is faced with trials and choices of a material nature. The second circle is Gwynvyd, a place of justified or reconciled spirits and is the union with god. The third circle is Ceugant where god alone exists. The three circles are also represented as suffering, change and choice or earth, the astral transition plane and the final home of the spirit. The spirit or soul is the eye of god which sees all things in its right form, place and time.
During this month the soul is raised to the Gwynvyd to be reminded of its purpose on earth. Jovyn was the gatekeeper who opens the door for Taliesin as he begins his journey through the underworld. Hence offerings to Jovyn of mugwort are often made and mugwort is hung over the door.