The answer to your question is rather simple. Clergy generally move in and out of the altar through the Royal Doors whenever they are doing something of great liturgical importance: placing the Gospel Book or Holy Gifts upon the Altar, performing a great censing, imparting God’s Word (reading the Gospel and offering a homily), issuing Christ’s blessing of “Peace,” and bringing out Holy Communion. At all other times the clergy use the two side doors when entering or exiting the Altar. Since the deacons utilize these side doors more often than priests because of the many litanies they perform, these doorways are affectionately known as “deacon’s doors.”
