Simplify the following text for a Grade 5 native English-speaking person.
Original: Year 1912 ( MCMXII ) was a leap year starting on Monday ( link will display the full calendar ) of the Gregorian calendar and a leap year starting on Sunday of the 13-day-slower Julian calendar .
Simplified: 1912 ( MCMXII ) was a leap year starting on Monday of the Gregorian calendar .