When we take the extra 14 days from the previous years combine them and add and extra day to the calendar called?

The extra 14 days combined with an additional day added to the calendar is associated with the transition from the Julian calendar to the Gregorian calendar. This adjustment was made to correct inaccuracies in the Julian system, which miscalculated the solar year. The term for this adjustment is often referred to as a "calendar reform," and the extra day added in a leap year is simply known as "Leap Day."