A thesis prospectus with initial bibliography, signed by the thesis adviser, is due in late April of your Junior year. A prospectus is a statement about your refined topic and your proposed argument. In addition, the prospectus should include a summary of what you hope to accomplish through further research.
The Head Tutor will alert you to the deadline at the start of the spring semester. You will also be expected to attend the second half of the Committee Degree Meeting in early May, and present your prospectus to the committee members. They will give feedback on the proposal, and make suggestions regarding sources, approaches, and people to consult.
If you revise your prospectus during the summer before the Senior year, you should make sure to present a revised copy to the Head Tutor.