366 ( CCCLXVI ) was a common year starting on Sunday of the Julian calendar , the 366th year of the Common Era (CE) and Anno Domini (AD) designations, the 366th year of the 1st?millennium , the 66th year of the 4th?century , and the 7th year of the 360s decade. As of the start of 366, the Gregorian calendar was 1 day ahead of the Julian calendar, which was the dominant calendar of the time.