On 12 August 1949 the four Geneva Convention were opened for signature. They, as we know, form the bedrock of what came to be known as international humanitarian law. They are also, to my knowledge, the first treaties ever that have become truly universal, meaning that all states in the world are parties to them (well, at least all states whose statehood is generally undisputed). The ICRC will mark the occasion with several events (more here and here).