The French maintain the D means “disembarkation,” still others say “debarkation,” and the more poetic insist D-Day is short for “day of decision.” When someone wrote to General Eisenhower in 1964 asking for an explanation, his executive assistant Brigadier General Robert Schultz answered: “General Eisenhower asked me to respond to your letter. Be advised that any amphibious operation has a ‘departed date’; therefore the shortened term ‘D-Day’ is used.” https://time.com/5599811/d-day-meaning/ 
Whether “D” stood for “day,” “disembarkation” or another word, the Allied invasion of Normandy was not the only D-Day in World War II. In fact, every amphibious assault  had its D-Day. “There were D-Days all through WWII where we performed that operation, where we landed on the beaches in Morocco and in North Africa in 1942 , we landed on the beaches in Sicily in 1943 ,” says the National WWII Museum’s Huxen.
So why when we say D-Day today, do we mean one particular day: June 6, 1944?
Among those landings, Normandy’s D-Day was the one that began the end of Nazi Germany’s control. It mattered so much that it kept a name that had originally been purposefully flexible. It signifies the day that the invasion will launch and puts all the timetables into play,” says Keith Huxen, Senior Director of Research and History at the National WWII Museum . The term H-Hour worked similarly, with “H” referring to the time on D-Day when the Allied troops hit the beaches. (H-Hour was 6:30 a.m.)  Had the Allied troops failed, Huxen says, “democracy would’ve been completely stamped out and crushed across continental Europe.”
During World War II (1939-1945), the Battle of Normandy, which lasted from June 1944 to August 1944, resulted in the Allied liberation of Western Europe from Nazi Germany’s control. Codenamed Operation Overlord, the battle began on June 6, 1944, also known as D-Day, when some 156,000 American, British and Canadian forces landed on five beaches along a 50-mile stretch of the heavily fortified coast of France’s Normandy region. The invasion was one of the largest amphibious military assaults in history and required extensive planning. Prior to D-Day, the Allies conducted a large-scale deception campaign designed to mislead the Germans about the intended invasion target. By late August 1944, all of northern France had been liberated, and by the following spring the Allies had defeated the Germans. The Normandy landings have been called the beginning of the end of war in Europe.
Normandy invasion facts
Austin Daily Herald
Press Association on MSN.com
By brilliantly coordinating their forces in the air, at sea, and on land, the Allies not only secured a foothold, they won the race to build up forces at the front. The result was overwhelming victory in Normandy 77 days later and a catastrophic defeat for Nazi Germany.
BBC History Magazine
Capa’s D-day shots reportedly  inspired director Steven Spielberg’s wrenching, realistic take on the landings in “Saving Private Ryan.” To pierce Hitler’s Atlantic Wall, more than 16,000 Americans would die and 40,000 would be wounded by the end of July.
Had the Allied troops failed, not only would democracy have been stamped
out and crushed across continental Europe, but America would have been next in
Sometimes war is the only language that socialists understand or to which they respond.
Another anniversary is that of the 30 years since the Tiananmen Square massacre in China. Yet commemorations took place in Hong Kong as the event remains widely unknown in mainland China.