The priority date is used by the Department of State to determine an individual’s place in line in the visa queue where there are a limited number of immigrant visas available in a given year. For family sponsored immigration, the priority date is the date that the petition is properly filed with U.S. Citizenship and Immigration Services (USCIS). The priority date for an immigrant petition that is based on employment is either the date the petition was filed with USCIS, or the date the labor certification application was accepted for processing by the Department of Labor.
USCIS, Visa Availability and Processing Time, www.uscis.gov/portal/site/uscis/menuitem.eb1d4c2a3e5b9ac89243c6a7543f6d1a/