The Sword in the Stone
A familiar motif in Arthurian legend. There are actually two swords that can be called the Sword in the Stone, and neither of them should be identified with Excalibur.
The earliest mention seems to be in Robert de Boron's Le Roman du Graal, in the "Merlin" section, which is believed to be dated around the year 1200.
Utherpendragon has died, leaving Logres without a king. Entor, Arthur's foster father, made his son Kay a knight at All Hallows; now at Christmas, they have gathered with the other knights, noblemen, and bishops to determin a new king. In a churchyard appears a stone, upon which there rests an anvil, and in the anvil a sword. None can draw the sword but the rightful king of Logres. Kay's squire, his foster brother Arthur, accidentally leaves Kay's sword behind during a tournament; Arthur goes searching for a sword, and finds the one in the churchyard. He pulls it, frees it, and gives it to Kay, who goes to Entor and claims that he pulled it. Entor doesn't believe him, but instead learns that Arthur is the one who pulled the sword. Entor presents Arthur to the nobles and bishops, who watch him perform the act again; still, they don't believe it. They make Arthur wait until Candlemas. Again, he pulls the sword. They make him wait until Easter. Again, he pulls it, and finally it is agreed that Arthur will be crowned at Pentecost.
This version is the most popular, and is later found in the Lancelot - Grail Cycle version, and later in Malory's Le Morte d'Arthur, nearly three hundred years later.
This sword is not Excalibur, though modern retellings sometimes makes that mistake. According to Malory, the sword of the stone was actually broken when Arthur fought King Pellinor early in his career. Merlin then took Arthur to a lake wherein dwelt the Lady of the Lake. Arthur rows to the island in the middle of the lake "as fair a place as any on earth, and richly beseen", and a damsel came to Arthur and gave him the sword. Now, as Geoffrey of Monmouth states that Excalibur came from Avalon, this island is likely meant to be Avalon.
According to the Lancelot-Grail version, there is a second Sword in the Stone, which appears towards the end of Arthur's reign. On Pentecost, there is seen in the river outside Camelot a stone with a sword sticking out of it. That same day, Galahad appears at court, sitting in the Siege Perilous. He has no sword, so Arthur suggests this new sword in the stone. Galahad pulls it, and the quest for the Grail begins.
Now, there are two things worth noting. First, it is made sure in the text that Galahad's appearance means the end of the enchantments and wonders of Britain--and thus the end of Arthur's reign. The sword in the stone motif then acts as bookends for Arthur's career.
Secondly, it is interesting to note the says upon which Arthur pulls the sword from the stone. The events begin with All Hallows, and pass through Christmas, Candlemas, Easter, and Pentecost. Oddly enough, these dates correspond not only to Christian holy days, but to Celtic and Germanic pagan holy days: All Hallows being Samhain, Candlemas being Imbolc, and Pentecost being around Beltane, and all three being major Celtic holy days; while Christmas and Easter are also the Germanic holidays of Yule and Ostara. The melding of medieval Christian and early Pagan holidays and their significances shine through in the Arthurian mythos, itself a mélange of Pagan and Christian themes. | 
