Operation Dynamo was the rescue operation implemented by the Royal Navy. It was co-ordinated by Vice Admiral Bertram Ramsay and his small team in Dover Castle. There, beneath the fortress, a network of tunnels deep within the cliffs became the nerve centre controlling the evacuation of Allied forces. From 19 May, realising that rescue by sea would be necessary, Ramsay and his staff at Dover were making plans and arranging for ships to evacuate the BEF. On 26 May they were ordered to put their plans into action. Things you need to know about the Dunkirk Evacuation Dunkirk evacuation - Wikipedia
At 18:27 to be precise. That said troops were already being evacuated up to four days earlier than Dynamo.
