February 14th is an unofficial, but widely celebrated in the professional world Computer Day. On February 14, 1946, the first really working electronic computer ENIAC I (Electrical Numerical Integrator And Calculator) was demonstrated to the scientific world and all those interested.
Interestingly, the work on the development of the first computer was sponsored by the American army, which needed a computer for military calculations, planning and programming. ENIAC I worked until 23:45 on October 2, 1955, and then was dismantled.
Of course, there were earlier computers, but these are all prototypes and experimental versions. For that matter, Babbage's Analytical Engine was the first computer in general... But ENIAC was the first computer that really worked on practical problems. By the way, it was from ENIAC that modern computers inherited the binary number system.