Video explains the meaning of 'D' in the phrase 'D-day' 
A video states that according to common explanations the 'D' in D-Day stands for 'designated'  'decision' or 'deliverance'. However, the term first used during World War II, was used to refer to the variable day when an attack was scheduled to happen. The term also provided the benefit of keeping the date a secret for as long as possible.