While sources aren’t in full agreement as to this exact date*, it is appropriate to recognize this milestone on Father’s Day since the microprocessors that dominate our lives today are all descendants of the Mark I inheriting some of its most fundamental conceptual design elements.

* June 21st is indirectly cited according to lab notes by Peter Van Roy and Seif Haridi in their magna opus “Concepts, Techniques, and Models of Computer Programming” (MIT Press, 2004 – page 36) whereas Wikipedia cites dates in April and June of 1949. But like much of the early History of Computing, we may never be able to reach “ground truth”.