There came a time when Arthur announced to Merlin that he needed a wife. He asked the old man for advice.

Now is there any that ye love more than another. Yea, said King Arthur, I love Guenever, the daughter of King Leodegrance of the land of Cameliard... And this damsel is the most valiant and fairest lady that I know living, or yet that I could find.
- Malory.

And so Merlin went to King Leodegrance and told him of Arthur's desire for his daughter. The King greeted the news with joy, and promised Arthur a gift - the Round Table. And so the King delivered his daughter Guenever to Merlin, and she and Arthur were married at the feast of Pentecost.

Then was the high feast made ready, and the King was wedded at Camelot unto Dame Guenever in the Church of Saint Stephen's with great solemnity
- Malory.

Then all the knights were sworn of the Round Table, and the ceremony was repeated each year on the day of Pentecost.