According to our encyclopedia entry on SOS, "mayday" is used as a distress signal for aviators because it approximates the French term "m'aider", meaning "come help me!" The term came into use between the years 1925-30.
"SOS" was selected as a distress signal by the International Radiotelegraphic Convention at London in 1912. It is expressed in Morse code as ". . . . . ." (three dots, three dashes, three dots), and was chosen not because it refers to any words, but because it is easy to transmit.