What is a Priority Date
You can file your adjustment of status application or consular process your case when there are "immigrant visa numbers" available from the Department of State for individuals in your preference category and born in your country of birth. The U.S. government limits the number of immigrant visas available to foreign nationals, based on country of birth and preference category. Every month, the State Department issues a "visa bulletin." This document tells us whether there will be visa numbers available the following month for each category or whether a backlog has developed and you must wait until the category is "current". Your place in the waiting line is determined by your "priority date". Your priority date is the date your labor certification was filed . (not the date it is approved.)
When can I file for my Green Card?
If, at the time your I-140 or I-130 is approved and your adjustment of status or consular process is ready to be filed, the visa bulletin indicates that the USCIS is accepting applications from persons of your nationality, in your preference category, and with your priority date, then you can file the adjustment application immediately. If there is a backlog, you must wait until USCIS is accepting applications with your priority date. So long as you have valid non-immigrant status, you can remain in the U.S. and wait. (You can be in H-1B status for up to 6 years). If your non-immigrant status expires before you can file your adjustment, you will probably be required to leave the U.S. and wait outside the U.S. for a visa number to become available. You will then apply for your legal permanent residency directly with the U.S. Consulate.