Simplify the following text for a Grade 6 native English-speaking person.
Original: The calendar has a regular year of 365 days divided into 12 months , and a leap day is added every four years to the month of February , making the average Julian year 365.25 days .
Simplified: The Julian calendar has a regular year of 365 days divided into 12 months with a leap day added to February every four years . The Julian year is , therefore , on average 365.25 days long .