The Druids – as they call their priests – know of nothing more sacred than mistletoe and the tree upon which it grows, especially when this is a winter oak. Almost naturally they frequent oak groves, and no sacred rite is performed without oak leaves, so that one might suppose that their name is derived from the Greek word drus for "oak tree". They regard everything that grows on this tree to be a divine gift and as a sign that this tree is chosen by God Himself. But one rarely finds mistletoe growing on oaks; and when it is found there, it is plucked with great ceremony on the sixth day after new moon, which indicates the beginning of their months and years, and also a period of 30 years, for the moon is then powerful enough but has not yet reached half its full size. In their language mistletoe is called the "cure-all." After performing proper offerings and feasts under the tree, they lead two white bulls there, whose horns are then wreathed. The priest, dressed in a white robe, climbs the tree and cuts the mistletoe with a golden sickle. It is caught in a white cloak. They then slaughter the sacrificial beasts, praying that the gods may be gracious to those to whom they have given this gift. Placed into drink, mistletoe is said to make all barren animals fertile and to be a remedy against all poisons.
From: Pliny the Elder: Natural history. Book XVI, chapter 95. English translation from Latin as quoted in German by Karl von Tubeuf in: "Monographie der Mistel" [Monograph of the mistletoe], Oldenbourg Verlag, Munich Berlin 1923; p. 19.