Of recent history one of the most important dates is 2004, the day a mega earthquake in the Indian Ocean triggered a tsunami that killed almost a quarter million people. It was one of the worst natural disasters in human history, and took a terrible toll on many of the nations of south Asia.
Another important date would be 1974, when India first succesfully tested a nuclear weapon. Pakistan concluded its first successful nuclear weapons test in 1998. The advent of nuclear weapons completely altered the political dynamic in the region.
The biggest events would be the creation of the various countries of South Asia. So you have 1947, which is when British India became independent and when it split into India and Pakistan. Then you have 1971, which was when Bangladesh split off from Pakistan and became independent.