Why are there one day in a leap year?

Question: Why are there one day in a leap year?
- Keywords:
  calendar,
  planet,
  year
Asked by E,J Molaga to Sonia on 6 Nov 2018.
- Sonia Lenehan answered on 6 Nov 2018:
  
  It is to keep us in sync with the seasonal year! wikipedia has a great page explaining this https://en.wikipedia.org/wiki/Leap_year

Question: Why are there one day in a leap year?