31 August is the day that both Peninsular Malaysia and Sabah saw the end of colonial government.
The Federation of Malaya comprising of 11 peninsular states, became independent on 31 August 1957. Thousands of Malayans stood in darkness for two minutes at midnight to mark the official handover from Britain with the singing of the national anthem and raising of the Flag of Malaya.
North Borneo, a British crown colony since 1946, attained self-governance on 31 August 1963. They adopted a four-striped flag (red, white, yellow, blue) with a green cantonment and a brown mountain, which was used until 1982. The current flag of Sabah, adopted in 1988 also has the mountain in the canton, but now in dark blue on a light blue background. The stripes are now blue, white and red.
So yes, these were the flags of the day. 😊 Happy Merdeka!