FYI
The date the visa petition was filed is known as your priority date.
visa petition is I-140.
Pl read below, the contents are from USCIS web site.
First, foreign nationals and employers must determine if the foreign national is eligible for lawful permanent residency under one of CIS' paths to lawful permanent residency.
Second, most employment categories require that the U.S. employer complete a labor certification request (Form ETA 750) for the applicant, and submit it to the Department of Labor's Employment and Training Administration. Labor must either grant or deny the certification request. Qualified alien physicians who will practice medicine in an area of the United States which has been certified as underserved by the U.S. Department of Health and Human Services are relieved from this requirement. You may wish to read more about this program.
Third, CIS must approve an immigrant visa petition, Form I-140, Petition for Alien Worker, for the person wishing to immigrate to the United States. The employer wishing to bring the applicant to the United States to work permanently files this petition. However, if a Department of Labor certification is needed the application can only be filed after the certification is granted. The employer acts as the sponsor (or petitioner) for the applicant (or beneficiary) who wants to live and work on a permanent basis in the United States.
Fourth, the State Department must give the applicant an immigrant visa number, even if the applicant is already in the United States. When the applicant receives an immigrant visa number, it means that an immigrant visa has been assigned to the applicant. You can check the status of a visa number in the Department of State's Visa Bulletin.
Fifth, if the applicant is already in the United States, he or she must apply to adjust to permanent resident status after a visa number becomes available. You may wish to read about application procedures on becoming a permanent resident while in the United States. If the applicant is outside the United States when an immigrant visa number becomes available, he or she will be notified and must complete the process at his or her local U.S. consulate office.
How Do I Apply for an Immigrant Visa Number?
You do not directly apply for an immigrant visa number. In most cases, your relative or employer sends a visa petition to the USCIS for you (the beneficiary) to become an immigrant. (Certain applicants such as priority workers, investors, certain special immigrants, and diversity immigrants can petition on their own behalf.) USCIS will tell the person who filed the visa petition (the petitioner) if the visa petition is approved. USCIS will then send the approved visa petition to the Department of State's National Visa Center, where it will remain until an immigrant visa number is available. The Center will notify you (the beneficiary of the application) when the visa petition is received and again when an immigrant visa number is available. You do not need to contact the National Visa Center, unless you change your address or there is a change in your personal situation that may affect your eligibility for an immigrant visa. You may contact the National Visa Center by writing to: The National Visa Center; 32 Rochester Avenue; Portsmouth, New Hampshire 03801-2909.
How Can I Find Out When an Immigrant Visa Number Will Be Available for Me?
Each approved visa petition is placed in chronological order according to the date the visa petitioin was filed. The date the visa petition was filed is known as your priority date. The State Department publishes a bulletin that shows the month and year of the visa petitions they are working on by country and preference category (see eligibility information above). You can estimate of the amount of time it will take to get an immigrant visa number by comparing your priority date with the date listed in the bulletin. For instance, suppose you look under your country and preference category, and see that the State Department is working on applications they received in May 1996. If your priority date is May 1998, then you may have to wait several more years for an immigrant visa number to become available. You may access the State Department Visa Bulletin at the State Department's Website, or you may call the Department of State Visa Office at (202) 663-1541, to learn which priority dates are currently being processed.