Each ship is assigned a Cruise Director 1 and these assignments will be in affect until the new ships come out. Brent is CD1 on the Magic; Christiaan is CD1 on the Wonder. Rachel is in the office helping with the new ships.
Clayton, Darren, Peter and Ray (and others) are Assistant Cruise Directors that step-up and do the cruise directors job when a CD1 is on vacation.
CD1 contracts are for 4 months - so most of the time you can count on your fingers when Brent and Christiaan will be on their ship.
Brent came on May 9th - 4 months would be around the 1st of Sept. Which means Brent should be back around the first part of Nov.
Christiaan came on in Mar and is off starting June 28th. Christiaan should be back on the Wonder around the end of Aug.
Special needs can change the time frame.