Ordinarily, anyone who is not the spouse or minor, unmarried child of a U.S. citizen must wait -- sometimes for many years -- for an immigrant visa to become available. Each month U.S. Citizenship and Immigration Services (USCIS) publishes a bulletin stating the Priority Dates that can be processed, which depends on the immigrant's visa category and country of chargeability (usually the country in which they currently hold citizenship).
Beginning in October 2015, these bulletins will list Filing Dates in addition to Final Action Dates.
For anyone waiting outside the U.S. (Consular Processing): If your petition's Priority Date (the day it was filed) falls on or before the listed Filing Date, you can assemble and submit your documentation at that time, though you still cannot receive a final decision until your Priority Date falls on or before the Final Action Date.
For anyone waiting inside the U.S. (Adjustment of Status): Your date for submitting an application is still controlled by the Final Action Date unless the Visa Bulletin indicates that you may instead use the “Filing Date” chart. This will vary from one month to the next, though it is permitted on the October 2015 Visa Bulletin. Note: if you are permitted to file early, you will also be able to obtain employment authorization and advance parole early. However, there is no guarantee that adjustment applicants will be able to use earlier Filing Date in future months, so those eligible in October should act promptly.
If you are within four years of your Priority Date coming current, it is worth consulting with an attorney to see if you might be able to file earlier than you had thought possible.