Simplify the following text for a Grade 6 native English-speaking person.
Original: Year 1915 ( MCMXV ) was a common year starting on Friday ( link will display the full calendar ) of the Gregorian calendar and a common year starting on Thursday of the 13-day-slower Julian calendar .
Simplified: 1915 ( MCMXC ) was a common year starting on Friday of the Gregorian calendar .