Having suffered heavily from Danish raids for the past five years, King Aethelred reputedly made the decision to have killed the Danes living in his kingdom. Known as the "St Brice's Day Massacre" because 13th November is this saint's feast day, it is unlikely to have extended into the area of the Danelaw itself, but was concentrated on frontier towns like Bristol, Gloucester, Oxford and London. It may have provoked King Sweyn of Denmark's invasion the following year.
Editors. "Æthelred orders massacre of Danes". The Literary Encyclopedia. First published 31 July 2012
[https://www.litencyc.com/php/stopics.php?rec=true&UID=1944, accessed 18 February 2019.]