Continental Sources English/English Latin Sources British Latin Sources Breton, Welsh & Cornish Sources
Before 500 Lucius Artorius Castus, Roman general in Britain 181 AD

6th c. Gallic Chronicles do not mention Arthur
Gildas, On the Ruin of Britain, 540, mentions a battle of Mount Badon of Romanized Christian Britons against Saxons, in 500 (?)--does not name Arthur
7th C.
673. Bede's Ecclesiastical History of the English Nation does not mention Arthur.
Y Goddodin, heroic poem of uncertain date referring to 7th c. events, compares a hero to Arthur (see 13th c.)
8th c.

"Nennius" History of the Britons, lists 12 battles of the general, Arthur, against the Saxons, including Badon

Nennius also describes the footprint of the soldier Arthur's dog, and Arthur's son's grave, as marvels worth a visit.

9th c.
890. Anglo-Saxon Chronicles (in Old English) do not menton Arthur.
Elegy of Geraint, heroic poem of uncertain date referring to events ca. 500, mentions emperor/warrior Arthur (see 13th c.)
10th c.

Saints' lives (Illtud, Gildas, Padarn, Cadoc, Carradog) depict King Arthur and his companions, Kei and Bedwyr, as boorish contemporaries of the saints.

970. Annales Cambriae (Welsh Annals): Arthur carried a cross at Badon in 517 and died with Medraut at Camlann in 540. 

12th c.
ca. 1100 A church portal in Modena depicts a captured woman being rescued by knights; the knights are labelled Artus de Bretannia, Gawain, Kay, the woman is Winlogee (Guinevere) 1125 William of Malmesbury in his Deeds of the English Kings mentions the stories of Arthur, the warrior victor of Badon, adding that much nonsense has been written about him..
1136 Geoffrey of Monmouth writes the History of the Kings of Britain, outlining King Arthur's reign, companions, etc. in detail.
1113. Canons of Laon visiting Cornwall find that the Cornish believe King Arthur will come to liberate them, and note that the Bretons have the same legend.
12th c.
1155 Geoffrey of Monmouth's History is translated into Norman French by Wace, who adds the story of the Round Table.

The story of Tristan is written down in French and German; one version places it a generation after Arthur, the other as contemporary with Arthur.

Chretien de Troyes writes romances about Arthur's knights, including Yvain, Erec, Lancelot, and Perceval. First mention of Grail.

Ulrich v. Zatzikhoven writes a Lanzelet drawing on a non-Chretien Lancelot tradition.

1190 Geoffrey of Monmouth's History is translated into English by Layamon (Lawman), who emphasizes Arthur's fairy connections.

1190-92 King Arthur's grave is discovered at Glastonbury and Gerald of Wales describes the discovery.
Marie de France translates into French Breton Lais, including Lanval, which implies she and her audience are with Arthur's court.
13th c. Chretien's work and the Tristan stories are translated into German, Old Norse, Italian etc.. The Grail story is expanded and rewritten, eventually including stories of Joseph of Arimathea and Christ. 

French prose romances are written incorporating the entire story of Arthur's birth, rise, reign, Grail quest, and death. A second growth spurt incorporates the Tristan story.

Welsh poetry collections, including Triads, Pa Gur, Y Goddodin, and Geraint's elegy (see above); also some manuscripts of the "Mabinogion" Arthurian stories
14th c.
English poems with Arthurian themes (Stanzaic Morte, Alliterative Morte, Sir Gawain and the Green Knight).

Winchester Round Table constructed.

Welsh story collections (known as the Mabinogion) include tales of Arthur, including the all-Welsh Culwych and Olwen and versions of Chretien's stories with alternate details.

Also the Book of Taliesin collects older Arthurian poems.
15th c. 
1470: Malory's Morte Darthur gives a coherent  form to the prose cyclical romances taking Arthur from birth to death.