The foundation of a good thesis is laid in the fall semester or sooner. A good thesis shows breadth of study and depth of insight. A senior cannot expect to read broadly in the relevant field after the fall semester; by then one must focus one's energies on the material that will appear in the final draft - on the writing and argument. One cannot afford to spend a leisurely month reading and thinking about the topic as a whole and the larger implications of one's research; instead, come spring, one will be overcome with the need to produce a finished piece of writing. This breadth can only be supplied in the relative calm of the preceding summer and fall term.
Similarly, the depth of one's argument comes from the repeated experience of making a generalization, stepping back to consider its inadequacies, and refining that position by further argument. This process of bold thesis-making and serious self-criticism necessarily takes time; it cannot be done in the month or two that is available in the spring. The result of a last-minute rush to write a thesis will be an error-ridden superficial work. It is important to begin writing as soon as possible. Thoughts that are written out are usually more coherent. It is also easier to improve on a thought when you can look back at it a week later and show it to your adviser for comment.