As you likely know – today is an extra day. The practice started over 2,000 years ago when Julius Caesar developed the Julian Calendar with the assistance of an Alexandrian astronomer, Sosigenes. Basically, the earth makes a full rotation around the sun every 365.25 days and did not fit neatly into a specific calendar or 24 hour cycle. So, it was decided that every four years they would add an extra day to make up for the extra quarter day. You can tell it’s a leap year if the last two digits of the year are divisible by four, except for century years. For a century year (e.g. 1900, 2000) to be a leap year it must be divisible by 400. So 2000 was a leap year, but 2100 will not be. Without leap years, eventually February would be a summer month in the northern hemisphere! Make the most of the extra day today.