Acting on advice by Alexandrian astronomer Sosigenes , Caesar did this to make up for the fact that the Earth's year is slightly more than 365 days. So, if calendar years contained 365 days they would drift from the actual year by about 1 day every 4 years. By adopting a leap year with an extra day every four years, the calendar year would drift much less. This Gregorian Calendar system is the one in common use today. That makes today the first leap day for a centurial year since year 1600 and the second such leap day of the Gregorian Calendar.
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