The Henry Project page for William gives the date of his death as "(probably) 17 December 942", adding that "a literal reading of both known manuscripts of the Planctus suggests that the event occurred on a Saturday (dies sexta or die sexto)", see http://sbaldw.home.mindspring.com/hproject/prov/willi000.htm
However, the sixth day of the week by medieval reckoning was Friday, not Saturday. This fits with William's death in 942 on 16 December (a Friday) as recorded in the obiturary of Jumièges, see (p. 422) https://archive.org/stream/recueildeshistor23bouq#page/422/mode/2up
Dudo gave 17 December ("XVI kalendas januarii"), although several manuscripts have this as 20 December ("XIII kalendas"). 17 December in 942 was a Saturday and 20 December a Tuesday. Dudo placed the event in 943, perhaps taking this mistake from Flodoard who had recorded it at the start of that year - for Flodoard the year began on Christmas day, and he probably heard the news at Reims shortly after 25 December.
There is no doubt Flodoard meant that the death of William had occurred at the start of his 943 since other events reported later in that year were consequences of it, particularly the fate meted out to the killer and the reconciliation of Louis IV and Arnulf of Flanders, with whom he had been angry over the murder, reported towards the end of the same year.
The correct date is most likely Friday 16 December 942, so that the two variant dates in copies of Dudo's work may both be misreadings of "XVII kalendas".