George Henry and E. A. Hornel: Druids Bringing In The Mistletoe (1890)
The painting follows the description of Pliny the Elder (1st century AD) in describing the ritual of oak and mistletoe:
HISTORICAL FACTS CONNECTED WITH THE MISTLETOE
The Druids—for that is the name [the Gauls] give to their magicians— held nothing more sacred than the mistletoe and the tree that bears it, supposing always that tree to be the robur [oak]. Of itself the robur is selected by them to form whole groves, and they perform none of their religious rites without employing branches of it; so much so, that it is very probable that the priests themselves may have received their name from the Greek name for that tree [δρῦς]. In fact, it is the notion with them that everything that grows on it has been sent immediately from heaven, and that the mistletoe upon it is a proof that the tree has been selected by God himself as an object of his especial favour.
The mistletoe, however, is but rarely found upon the robur; and when found, is gathered with rites replete with religious awe. This is done more particularly on the fifth day of the moon, the day which is the beginning of their months and years, as also of their ages, which, with them, are but thirty years. This day they select because the moon, though not yet in the middle of her course, has already considerable power and influence; and they call her by a name which signifies, in their language, the all-healing. Having made all due preparation for the sacrifice and a banquet beneath the trees, they bring thither two white bulls, the horns of which are bound then for the first time. Clad in a white robe the priest ascends the tree, and cuts the mistletoe with a golden sickle, which is received by others in a white cloak. They then immolate the victims, offering up their prayers that God will render this gift of his propitious to those to whom he has so granted it. It is the belief with them that the mistletoe, taken in drink, will impart fecundity to all animals that are barren, and that it is an antidote for all poisons. (Karl Friedrich Theodor Mayhoff, trans.) (source)