In September 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 Non-urgent content must not be published.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?