Short answers, pulled from the story.
The Battle of Hong Kong was fought from the 8th to the 25th of December 1941. It began on the same morning as the attack on Pearl Harbor and ended on Christmas Day with the surrender of Governor Young and General Maltby to General Sakai at the Peninsula Hotel.
British defence studies had concluded well before 1941 that Hong Kong would be extremely difficult to defend. Japanese control of Canton, Hainan Island, French Indo-China, and Formosa had effectively surrounded the colony before a shot was fired. The British military also grossly underestimated Japanese capabilities, and warnings were dismissed as unpatriotic. The garrison's reinforcements, including the Canadian C Force, arrived without vehicles, mortar ammunition, or adequate field training.
Canada sent C Force to Hong Kong, comprising the Royal Rifles of Canada and the Winnipeg Grenadiers, totaling 1,975 personnel, who arrived on the 16th of November 1941. Few had field experience, and their vehicles never arrived, having been diverted to Manila. C Force suffered 23 officers and 267 other ranks killed or died of wounds in the battle, and 267 of those captured subsequently died in Japanese prisoner of war camps.
Japanese forces committed widespread atrocities throughout the battle. These included the murder of wounded soldiers and medical staff at the Salesian Mission dressing station on the 19th of December, the massacre of nurses and patients at St. Stephen's College field hospital on the night of the 24th of December, the beheading of prisoners at Deepwater Bay, and the massacre of around thirty civilians on Blue Pool Road on the 22nd of December. An estimated 10,000 Hong Kong civilians were executed during the subsequent occupation.
Sergeant-Major John Robert Osborn of the Winnipeg Grenadiers was posthumously awarded the Victoria Cross for smothering a Japanese grenade with his body on the 20th of December 1941. Colonel Lance Newnham, Captain Douglas Ford, and Flight Lieutenant Hector Bertram Gray each received the George Cross for refusing to disclose escape plans under Japanese torture before being shot. Captain Mateen Ansari of the 5/7th Rajputs received the George Cross for resisting Japanese coercion for over five months before being executed by beheading on the 20th of October 1943.
Black Christmas refers to the 25th of December 1941, the day Governor Young and General Maltby surrendered Hong Kong to General Sakai at the Peninsula Hotel. The term marks both the surrender itself and the beginning of a Japanese occupation that lasted until the summer of 1945, during which an estimated 10,000 civilians were executed.