The end of June each year. The ADS login page will have a message that notifies users of the actual date of the rollover. The academic year in ADS is from July 1 through June 30.
In June, it is important note which academic year ADS currently reflects. Some users are eager to enter new residents/fellows. New residents/fellows entered prior to the academic year rollover that are arriving in the future should be entered with the status "Not in Program Yet".