Dag Hammarskjold ( Swedish : Dag Hjalmar Agne Carl HammarskjÃ ld ) ( 29 July 1905 -- 18 September 1961 ) was a Swedish diplomat and the second Secretary-General of the United Nations . Dag Hjalmar Agne Carl HammarskjÃ ld ( 29 July 1905 â `` 18 September 1961 ) was a Swedish diplomat , economist and author . He was the second Secretary-General of the United Nations .