All students are required to declare a concentration midway through their fourth semester (by October 15 in the fall and by April 1 in the spring). The fall Concentration Fair, as well as other sophomore-specific events, are designed to help you get a jumpstart on the declaration process. Your sophomore advisor can also help you think broadly and carefully about your concentration options.
Before declaring your concentration in ASK, you should speak with that program's concentration advisor. Consult the website of the department that houses your concentration for information about its advising system and whom to contact. Particularly in large concentrations, try to see concentration advisors early in the spring semester. Their schedules are very full, and you want to make sure that you meet with them prior to the deadline for submitting your concentration declaration in ASK.