What historical event led to the first instance of the "leap year" concept in the Julian calendar?

Although leap years are a common part of our modern calendars, with an additional day added to February every four years, the concept has its origins in ancient times. The Julian calendar, introduced by Julius Caesar in 45 BCE, was designed to better align the calendar year with the solar year. Caesar, advised by the astronomer Sosigenes of Alexandria, implemented the leap year concept to address the discrepancy caused by the solar year being approximately 365.25 days long. This adjustment aimed to keep festivals and seasons in step with the progression of time, correcting the drift that had occurred in previous calendars and laying the foundation for later refinements in the Gregorian calendar.