Answer: The optimum time to begin your search is just after Labor Day, when the lateral hiring season commences. During the summer months, law firm staff is focused on their summer associate class and many decision makers are also on vacation. This is not to say that no hiring occurs during the summer; however, it is typically the slowest time of year for lateral hiring, so is not necessarily the best time to begin your job search.
At the end of summer, firm administration is often focused on OCI, or on-campus recruitment of the next year's summer associate class. From there, in September, all attention turns to lateral hiring and this generally remains the focus until the start of the following summer (i.e., for a period of about 9 months).
Keep in mind that, even if you are not looking to make an immediate move, job searches can take a long time. I generally advise my candidates to budget at least 6 to 9 months for their job search. Of course things can happen more quickly, but generally this is the length of time involved in a thorough job search. Factors to consider include current economic conditions and resulting practice area demand, as well as overall fit.
Lastly, by September, many associates want to wait and collect their end of year bonuses before moving to a new law firm. This should not stop you from beginning a job search. As I mentioned above, the hiring process generally takes several months. Even if things do move more quickly than expected, firms are typically willing to let new lateral hires delay their start date until after the New Year. However, there are exceptions, especially if the hiring need is urgent.