Original sentence: In solar calendars, this is often done by adding to a common year of 365 days, an extra day (leap day or intercalary day) about every four years: this makes a leap year of 366 days.This is usually 12 months, but a 13th month (an intercalary or embolismic month) is added to the year every two or three years
Edited sentence: In solar calendars, this is often done by adding to a common year of 365 days, an extra day ("leap day" or "intercalary day") about every four years: this makes a leap year of 366 days.This is usually 12 months, but a 13th month (an "intercalary" or "embolismic" month) is added to the year every two or three years