A Day or A-Day may refer to:
- 20 October 1944, the day the Leyte Island Operation began
- A Day (film), a 2017 South Korean film
- A-Day (University of Alabama), an annual college football exhibition game in the United States
- A-day (6 April 2006), the commencement of pension tax simplification in the United Kingdom
- "A Day", a 2017 episode of the American Netflix series Love