The Question and Answer section for Le Morte d'Arthur is a great resource to ask questions, find answers, and discuss the novel.
Lancelot has had a love affair Guinevere the Queen (Arthur's wife). This makes things rather awkward between Arthur and Lancelot.
Arthur's birth was kept a secret in order to protect him. Uther had also made a promise to Merlin that once the child he foresaw was born, he would put the infant in Merlin's care.
I don't see them celebrating. Are you referring to Sir Gawain and the Green Knight?