Her on þissum geare se cyng gerædde 7 his witan. þæt man sceolde gafol gyldon þam flotan. 7 frið wið hi geniman wið þon þe hi heora yfeles geswican sceoldan. Ða sende se cyng to þam flotan Leofsig ealdorman. 7 he þa þæs cynges worde 7 his witena grið wið hi gesætte. 7 þet hi to metsunge fengon 7 to gafle. 7 hi þa þæt underfengon. 7 him man þa geald .xxiiii. þusend punda. Ða on gemang þysum ofsloh Leofsig ealdorman Æfic þæs cynges heahgerefan. 7 se cyng hine ða geutode of earde. And þa on þam ilcan lengtene com seo hlæfdige Ricardes dohtor hider to lande. On ðam ilcan sumera Ealdulf arcebiscop forðferde. 7 on ðam geare se cyng het ofslean ealle ða Deniscan men þe on Angelcynne wæron on Bricius messedæg. forþon þam cynge wæs gecydd þæt hi woldon hine besyrewian æt his life. 7 syððan ealle his witan. 7 habban syþðan his rice.
[In this year the king and his advisers decided that they should pay tribute to the [Danish] fleet, and make peace with them on condition that they should cease their harmful attacks. Then the king sent Ealdorman Leofsige to the fleet, and at the command of the king and his advisers he arranged that there should be a peace-settlement with them, and they should receive provisions and tribute. They accepted that, and they were paid 24, 000 pounds. Then in the middle of this Leofsige killed Æfic, the king's high-reeve, and the king exiled him from the country. And that same spring the Lady, daughter of Richard [Duke of Normandy], came here to this country. That same summer Archbishop Ealdulf died. And in that year the king ordered to be killed all the Danes who were among the English people, on St Brice's Day, because the king was told that they were plotting to take his life and then those of all his advisers, and after that have his kingdom.]
That's all the Chronicle tells us - the order, and why it was given, but not how and where it was carried out. The armies of the Danish king Svein Forkbeard and other Scandinavian leaders had been raiding England on and off for the past decade, so Æthelred could be forgiven for being desperate and a bit paranoid - but St Brice's Day did little to help, and probably made things worse. We know from a contemporary source that in Oxford - a border town where the Danish residents were as likely to be traders as Viking warriors - a group of Danes took refuge in St Frideswide's church (on the site of what is now Christ Church Cathedral), and the church was burned down by their pursuers. The order was impractical from the outset, because in the early eleventh century it would have been impossible to kill 'all the Danes among the English'; there were just too many of them, and the order certainly could not have been carried out in the north and east of England, a mixed population with many people of Danish or part-Danish descent (just ten years later this part of England would reject Æthelred and accept Svein Forkbeard as king - can't imagine why!). Perhaps the idea was not really to encourage people to kill their neighbours but to force them to choose sides, to affirm that they were part of the Angelcynn and not sympathetic to the Danes. But in practice the order must have been directed at small communities settled within towns like Oxford, which makes it a particularly cruel and ineffective idea - this was the kind of bad counsel which got Æthelred his nickname 'Unready' ('ill-advised'). The marriage which took place in that year was not well-omened: 'the Lady' mentioned by the Chronicle, the formidable Emma, came to England to marry Æthelred, and the following year gave birth to his son Edward (the future Confessor) - but she ended up fifteen years later married to her husband's enemy, Cnut. You can see what a chaotic state the English nobility were in from that odd reference in the Chronicle to Ealdorman Leofsige killing Æfic and being exiled. The atrocities of the Viking Age were not, by any means, all on the Viking side.
Apart from the events in Oxford, it would be difficult to assess the impact of St Brice's Day if it were not for the historian Henry of Huntingdon, writing about 130 years later, reporting what he had heard about that day:
King Ethelred’s pride increased and his faithlessness grew: in a treacherous plot, he ordered all the Danes who were living peacefully in England to be put to death on the same day, namely the feast of St Brice. Concerning this crime, in my childhood I heard very old men say that the king had sent secret letters to every city, according to which the English either maimed all the unsuspecting Danes on the same day and hour with their swords, or, suddenly, at the same moment, captured them and destroyed them by fire.
Historia Anglorum, ed. and trans. by Diana Greenway (Oxford, 1996), p. 341.
Henry was a child in the 1090s, so the people he heard talk about this during his childhood were (unless they were very old indeed) probably not eyewitnesses, but had perhaps heard about the massacre from their parents. The story sounds exaggerated, but is perhaps more interesting for being so; it suggests that the 'massacre' was remembered in the East Midlands, where Henry grew up, as an event of greater significance than the bald record of the Anglo-Saxon Chronicle would suggest. There are other twelfth-century references to it which give a similar impression: William of Malmesbury tells how the sister of Svein Forkbeard was supposed to have been among those killed, together with her husband and child:
[When Svein invaded England] his chief purpose was to avenge his sister Gunnhild. Gunnhild, who was a woman of some beauty and much character, had come to England with her husband the powerful jarl Pallig, adopted Christianity, and offered herself as a hostage for peace with the Danes. [Æthelred's chief adviser] Eadric in his disastrous fury had ordered her to be beheaded with the other Danes, though she declared plainly that the shedding of her blood would cost all England dear. And for her part, she faced death with presence of mind; she never grew pale at the prospect, nor did she change expression after death, even when her body was drained of blood, though her husband had been killed before her eyes, and her son, a very likely child, pierced by four lances.
Gesta Regum Anglorum, ed. and trans. by R. A. B. Mynors, R. M. Thomson and M. Winterbottom (Oxford, 1998) p. 301.
There may a grain of truth in this story; Pallig was a Danish warrior who had been recruited to fight for Æthelred and then turned against him in 1001, so Æthelred might well have taken the opportunity of St Brice's Day to kill him. And it's not impossible that he was married to Svein's sister and that they were both killed in 1002. But Gunnhild's bravery and her prophecy - 'the shedding of her blood would cost all England dear' - are pure legend, and interesting for that reason. Vengeance for such a death is a noble motive for an invasion, and avenging his valiant sister casts Svein in a more positive light than English sources usually do. There are multiple stories which say the Danes invaded England to avenge the death of an innocent family member, and in my view they were originally part of a kind of mythologising and justifying of the Danish conquest which went on in the early eleventh century (whether a later writer like William of Malmesbury intended this by repeating them or not). It's attractive to give a story a human motive like love or revenge, part of the process which turns historical events into legend and memory. Gunnhild's bravery on St Brice's Day is the stuff of such legend, and who knows whether Svein and Cnut really thought Æthelred was responsible for her death; but it's a nice irony that some years later, when Cnut married Æthelred's widow, 'the Lady' Emma who came to England in the year of St Brice's Day, they named their daughter Gunnhild.
How a later medieval artist imagined Svein's invasion (BL Harley 2278, f. 98v)