Law schools begin accepting applications between September 1st and October 1st of the year before the applicant intends to begin his or her 1L year.  Most schools -- including Yale, Stanford, Columbia, Chicago, and Duke -- allow applicants to submit their packets on September 1st.  Others, like Harvard and Georgetown, open their admission windows at various points during the month of September.  Berkeley will only begin accepting applications on October 1st. 

The final day on which one can apply also varies, but for most schools it's either February 1st or February 15th.  Be sure to check the admissions website for every school to which you apply for its particular submission windows.

Regardless of when any particular school allows you to subit your application, remember:  APPLY AS EARLY AS POSSIBLE.  More on this soon...