In September 2021 the plans for the Queen's death were leaked https://www.politico.eu/article/queen-elizabeth-death-plan-britain-operation-london-bridge/
Read some key quotes from the article.
"The U.K. government website — GOV.UK — will display a black banner at the top. All government departmental social media pages will also show a black banner and change their profile pictures to their departmental crest. Non-urgent content must not be published. Retweets are explicitly banned unless cleared by the central government head of communications.
The royal family will announce plans for the queen’s funeral, which is expected to be held 10 days following her death."
10 KNOWN upcoming days of "darkness"
"Internally, the day will be referred to as “D-Day.” Each following day leading up to the funeral will be referred to as “D+1,” “D+2” and so on."
Could this be what D5 refers to?
Other D5 meanings include: An avalanche, December 5th...
D-Day In the hours after the queen’s death, a “call cascade” will take place informing the prime minister, the cabinet secretary (Britain’s highest-ranking civil servant) and a number of the most senior ministers and officials. The PM will be informed by the queen’s private secretary, who will also tell the Privy Council Office, which coordinates government work on behalf of the monarch.
Internally, the day will be referred to as “D-Day.” Each following day leading up to the funeral will be referred to as “D+1,” “D+2” and so on.
The royal household will issue an “official notification” delivering the news to the public.
A call script for departmental permanent secretaries outlining how to break the news to their ministers, seen by POLITICO, instructs them to say: “We have just been informed of the death of Her Majesty The Queen.” Ministers will be told that “discretion is required.”
Ministers and senior civil servants will also receive an email from the cabinet secretary, a draft of which reads: “Dear colleagues, It is with sadness that I write to inform you of the death of Her Majesty The Queen.”
Upon receipt of this email, flags across Whitehall will be lowered to half-mast. The aim is that this can be done within 10 minutes.