Because its seeds are said to be dropped by the mistle-thrush, which feeds on its berries.
Why was the mistletoe held sacred by the Druids?
Because they had an extraordinary reverence for the number three, and not only the berries, but the leaves of the mistletoe, grow in clusters of three united on one stalk. Its growing upon the oak, their sacred tree, was doubtless another cause of its veneration.
We read of a celebrated oak at Norwood near London, which bore mistletoe, "which some people cut for the gain of selling it to the apothecaries of London, leaving a branch of it to sprout out; but they proved unfortunate after it, for one of them fell lame, and others lost an eye. At length, in the year 1678, a certain man, notwithstanding he was warned against it, upon the account of what the others had suffered, adventured to cut the tree down, and he soon after broke his leg."—Camden.
Mr. Brand, however, thinks that mistletoe was never put up in churches but by mistake or ignorance of the sextons: it being a heathenish and profane plant, and therefore assigned to the kitchen. Mr. Brand made many diligent inquiries after the truth of this point. He learnt at Bath that it never came into churches there. An old Sexton at Teddington told him that mistletoe was once put up in the church there, but was by the clergyman immediately ordered to be taken away.
Why was the boar's head formerly a prime dish at Christmas?
Because fresh meats were then seldom eaten, and brawn was considered a great delicacy. Holinshed says, that "in the year 1170, upon the day of the young prince's coronation, King Henry I. served his sonne at table as server, bringing up the boar's head with trumpets before it, according to the manner." For this ceremony there was a special carol. Dugdale also tells us, that "at the inns of court, during Christmas, the usual dish at the first course at dinner was a large bore's head, upon a silver platter, with minstralsaye." In one of the carols we read that the boar's head is "the rarest dish in all the londe, and that it has been provided in honour of the king of bliss."