When should I take my senior seminar?

You can take your senior seminar any semester after you have completed your core requirements (Principles, Intermediate Micro, Intermediate Macro and Econometrics). Please note that priority in registration is given to seniors, and juniors are only allowed to sign up during the second week of the semester (see the Senior Seminar Web page for details).
Although you can take a seminar without having taken any electives, seminars frequently rely on material from the elective courses. You should only register for a seminar in which you are already familiar with the background material. For example, a seminar that focuses on exchange rate crises should be taken only if you have taken a course in international monetary theory or finance.
A final point to consider when choosing your senior seminar is whether or not you need a recommendation letter from someone in the economics department. If you would like a recommendation letter from a faculty member of the economics department for the purposes of admission to a graduate school, then it is recommended that you take your senior seminar no later than the fall semester of your senior year.