Prior to your Senior year
You should begin thinking about and planning your thesis as soon as possible, but certainly during your Junior year. Before you apply to the History Honors program you must find a faculty member who approves your proposed topic and agrees to work with you to develop it and to advise your project throughout your Senior year. Typically you will already have taken courses with this professor and are proposing a topic within that professor's field of specialization.
By the end of the winter term of your Junior year you should have a thesis topic and arranged to work with a faculty advisor. Your proposal is due by the end of the second week of the spring term (by 4 pm on April 5, 2019). You may begin researching and refining your topic during the summer. Typically you will return to campus in the fall term with a much clearer understanding of the subject of your research, the questions your research will answer and what these answers are likely to be.