First, we prefer to worship out-of-doors; as we are a nature religion, we believe that no building can give us the sense of contact we feel when our bare feet rest upon the Mother Earth, or the sense of blessing we feel when a breeze caresses our skin. To worship is to enter the presence of God or the Gods or Universe; so for us, worshipping inside a building is more like leaving the Divine Presence.
Given that, the best place to do this would be in a quiet spot, not frequented by picnickers, traffic, or smog; a place remote enough to minimize the distractions of the man-made world. These days, there are few areas that offer much along these lines, but there are still woods to be found, and when we can, we worship there. The fact that trees are important symbols for us only enhances the appeal of the woods.
So, why at night? There are several reasons: after dark, people go indoors & get quiet - this enhances the peacefulness (as almost everyone knows) and the serenity of our outdoor communion, which is important to us; also, the moon for us is a symbol of the Goddess, and our rituals are synchronized with the lunar phases - this makes nighttime appropriate to us, for night is when the moon is most prominent.
There is one more reason we hold Circle (our worship) in the woods at night - in practice, the more isolated we are, the safer we are. In our Circle site in Lithonia, we were too close to go unheard, and too far to be understood; the sad result of that incident was that we were driven away by wild rumors and fears of the unknown.