It’s Arthur and Guinevere’s wedding feast, just a couple of years after he pulled the sword from the stone. Nothing else super weird has happened since then. A bunch of rebel lords challenged his right to rule, calling Merlin and his sword a fraud, and the succession was resolved with bigger army diplomacy, which is a totally standard way for a medieval king to spend the first couple of years of their reign. Now that his rule is consolidated, Arthur is looking forward to hopefully at least a couple of years of feasting and jousting and generally enjoying kingship without having to kill anyone over it.
He’s appointed Kay, his adoptive older brother, as his seneschal, so Kay would’ve been in charge of organizing the wedding reception. Kay is often depicted as a brute by Le Morte d’Arthur because even though Thomas Malory is English, his work is a product of the continental tradition, and the French for some reason thought Kay was two parts beatstick to one part buffoon, but apparently he’s perfectly capable of managing a giant wedding reception all by himself. Like, Arthur is undisputed king now and the text mentions some podunk knights here who will get famous later but right now are total nobodies, so it seems like the entire noble class of England was invited. Probably Kay was also in charge of security, so maybe that’s where the beatstick thing comes in, but his main responsibility would’ve been arranging the food and gifts for the guests (when you’re king, you give gifts to the guests at your wedding reception instead of the other way around), so he’s apparently a pretty competent manager. Take that, the continental tradition.
It’s in the middle of the feast on the first day of the celebration when a white hart comes running into the great hall, leaping over tables and running between the benches. Before anyone can do anything about that, a white dog comes running in chasing after the white hart. Before anyone can do anything about that, no less than sixty black dogs come charging in, completely overrunning the great hall, getting up on all the tables and knocking over all the wine goblets and eating all the best parts of the duck.
The white dog bites down on the white hart’s flank, but the hart knocks the dog off and into the lap of one of the knights at the feast. That knight grabs the dog and skedaddles while everyone else is too busy trying to clear the three score black dogs away.
Then a lady rides into the great hall, like, on a horse. She must’ve seen the knight who stole the dog on the way out, because as soon as she comes in, she shouts – so as to be heard over the black dogs, who still haven’t been evicted – “hey, that was my dog, someone get me my dog back!” Before anyone can ask her what’s with the hart or if she happens to be the owner of the sixty black dogs that just ruined the feast and if maybe she can get them outside, a knight in black armor riding a full-on warhorse charges into the great hall, hefts her off the saddle, and carries her away.
And Arthur’s like “what the fuck.”
And Merlin says “it’s a quest hook, Your Majesty. You’re supposed to send knights to figure out what the fuck.”
So Merlin helps Arthur pick some knights out to chase down the stag, the dog, and the lady, and slowly Arthur realizes that he’s being given a tutorial and this is his life now.
Malory never says one way or another, but I think we can safely assume that Kay was blackout drunk for all of this.