The actual date could be the 26th September, the day that the Japanese were ordered back from Imita Ridge, just south of Ioribiawa, their first retreat of the war. This was actually their first defeat. A national day or rememberance would be appropriate here, but it will never happen. Firstly, Anzac Day holds the national mythology, not Kokoda. Secondly, the New Guinea campaign was a side show, like Gallipoli, but unlike Gallippoli, Kokoda has to contend with how the Americans won the war in the Pacific.
After showing how the Japanese can be defeated, the Australian troops were sidelined by MacArthur, he distrusted any army that did not have the death penalty at the core if its military justice system, he also disliked the lack of formalty and generally egalitarian nature of the Australian troops, believing it to be unprofessional.
MacArthur also had a press corps that was greater in number than the Australian General Staff, how could any word of such a sideshow as Kokoda, which was in his view of no real importance be given any credence anywhere other than in Australia? (It is interesting to note that Chester Nimmitz had an entire press corps of 2, and he directed the Island hopping of the Marines that led to the ultimate defeat of japan, but it was MacArthur who accepted the Japanese surrender on the Missouri. Unbelievable, but true. To be fair though, MacArthur's real accomplishments were formidable, but nowhere near the mythological deeds his press corps made them out to be )
Under those circumtances, with a government that was enarmoured of MacArthur's myths, Kokoda was to be ignored for the achievement it was, not lauded for what it could have been, if MacArthur had been in charge, not Blamey.
BTW, the controversy of the time, Blamey calling the troops cowards and sacking Rowell was more about appeasing MacArthur's ego than anything else. MacArthur dismissed Australian reports from Kokoda and refused to aid the Australian troops believing that the main assualt on Port Moresby was coming from sea borne landings. Worse still, from MacArthur's view, the US Marines were landing on Guadalcanal, thus upstaging anything that MacArthur was doing. Consequently, for the media of the day, the main action was with the US Marines on Guadalcanal between August 1942 to February 1943 not Australian troops in New Guinea in September 1942. MacArthur certainly did not want to be seen as being behind the action, so it was not until the joint assault on Buna and Gona that the American troops were really into the action, too late for MacArthur to gain too much mileage from it.
Anyway, all of that is why Australia will never celebrate a Kokoda Day, not because it was a small achievement, but because it had no hysterically positive press coverage like Gallipoli. It had no CW Bean to write it up as myth either.