In 1886, Myanmar (then called Burma) came under British control. From the earliest days of colonisation, there was a strong feeling of resentment against the rule of the British.
During World War II, Some Burmese saw the rise of Japan as an opportunity to gain independence from Britain. Aung San was a prominent figure in the independence movement, but he had been exiled to China. He collaborated with Japan, having been convinced they would make Burma an independent nation if they helped drive out the British.
With Burmese nationalist support, Japan took control of Burma in 1942. However Aung San came to realise that the Japanese had no intention of giving independence to Burma and on March 23rd 1945, he led the Burmese army in a rebellion against the Japanese that helped the Allied forces remove the Japanese from the region.
Not surprisingly, the key event on this day is a large parade by the Tatmadaw, the Myanmar military in Naypyidaw, the capital of Myanmar.
Originally this holiday was called Resistance Day, but it was renamed to Armed Forces Day by the Tatmadaw.