Cryptologic History Calendar
1969: 1st Mesage Sent on DoD ARPANET
The first message sent over the ARPANET was from Leonard Kleinrock’s UCLA computer by student programmer Charley Kline at 10:30 pm on October 29, 1969, to the second node at Stanford Research Institute’s computer in Menlo Park, California.
The message was simply “Lo.”
"The message text was the word login; the l and the o letters were transmitted, but the system then crashed. Hence, the literal first message over the ARPANET was lo. About an hour later, having recovered from the crash, the SDS Sigma 7 computer effected a full login." (Wikipedia)