This story is part of the King Arthur unit. Story source: King Arthur: Tales of the Round Table by Andrew Lang and illustrated by H. J. Ford (1902).
THE COMING OF THE HOLY GRAAL
It was on the eve of Pentecost that all the Knights of the Table Round met together at Camelot, and a great feast was made ready for them. And as they sat at supper they heard a loud noise, as of the crashing of thunder, and it seemed as if the roof would fall on them.
Then, in the midst of the thunder, there entered a sunbeam, brighter by seven times than the brightest day, and its brightness was not of this world. The Knights held their peace, but every man looked at his neighbour, and his countenance shone fairer than ever it had done before.
As they sat dumb, for their tongues felt as if they could speak nothing, there floated in the hall the Holy Graal, and over it a veil of white samite, so that none might see it nor who bare it. But sweet odours filled the place, and every Knight had set before him the food he loved best, and after that the Holy Vessel departed suddenly, they wist not where.
When it had gone their tongues were loosened, and the King gave thanks for the wonders that they had been permitted to see.
After that he had finished, Sir Gawaine stood up and vowed to depart the next morning in quest of the Holy Graal, and not to return until he had seen it. "But if after a year and a day I may not speed in my quest," said he, "I shall come again, for I shall know that the sight of it is not for me." And many of the Knights there sitting swore a like vow.
But King Arthur, when he heard this, was sore displeased. "Alas!" cried he unto Sir Gawaine, "you have undone me by your vow. For through you is broken up the fairest fellowship, and the truest of knighthood, that ever the world saw, and when they have once departed they shall meet no more at the Table Round, for many shall die in the quest. It grieves me sore, for I have loved them as well as my own life." So he spoke, and paused, and tears came into his eyes. "Ah, Gawaine, Gawaine! You have set me in great sorrow."
"Comfort yourself," said Sir Lancelot, "for we shall win for ourselves great honour, and much more than if we had died in any other wise, since die we must."
But the King would not be comforted, and the Queen and all the Court were troubled also for the love which they had to these Knights.
Then the Queen came to Sir Galahad, who was sitting among those Knights, though younger he was than any of them, and asked him whence he came, and of what country, and if he was son to Sir Lancelot. And King Arthur did him great honour, and he rested him in his own bed.
And next morning the King and Queen went into the Minster, and the Knights followed them, dressed all in armour, save only their shields and their helmets. When the service was finished the King would know how many of the fellowship had sworn to undertake the quest of the Graal, and they were counted and found to number a hundred and fifty.
They bade farewell, and mounted their horses, and rode through the streets of Camelot, and there was weeping of both rich and poor, and the King could not speak for weeping. And at sunrise they all parted company with each other, and every Knight took the way he best liked.