Your priority date is your place in line for a green card.
It matters because, for most family and employment categories, there are more people waiting than there are green cards available annually, so the government uses this date to decide when it’s your turn.
Unlike most family and employment based categories, people who are immediate relatives of U.S. citizens don't wait in this line at all, because their visas are always available.
Which date becomes your priority date
For an employment-based case that needs a labor certification (PERM), it's generally the date the Department of Labor accepted that PERM application for processing. For employment-based cases that don't require the lengthy PERM process, like EB-1 or a National Interest Waiver, it's the date your I-140 was properly filed with USCIS.
For a family-based case, it's the date your relative's Form I-130 was properly filed with USCIS.
Where to find your priority date
Look at your Form I-797 Notice of Action for the I-130 or I-140. The priority date is printed on it. If you're not sure which notice you have, your attorney or your employer's immigration team can point you to it. Be sure to save a copy somewhere safe, that you control outside of your employer.
How it connects to the Visa Bulletin
Every month, the State Department publishes the Visa Bulletin with cutoff dates for each category and country. "C" means current, so numbers are available to everyone in that category, and "U" means unavailable for now.
If your priority date is earlier than the date listed for your category and country of birth, it is your turn to either file, or have your application adjudicated (this is dependent upon which date USCIS is accepting filings for any given month).
USCIS uses the bulletin to decide when you can file Form I-485, and a visa has to be available both when you file and when USCIS approves the case.
Country of birth (not country of citizenship) matters because there are limits on how many green cards can go to people from any one country, which is why the line for India and China moves so much more slowly.
Can you keep it if you change jobs?
Yes. Once your I-140 is approved, you can carry that priority date to a later EB-1, EB-2 or EB-3 petition, even with a new employer. The exceptions are if USCIS revokes the approval for fraud or willful misrepresentation, a problem with the labor certification, or a material error. If an I-140 petition is denied, then the priority date is not set.
Where Connect by InHouse Core fits
Connect by InHouse Core has a priority date tracker that tracks the Visa Bulletin next to your own priority date, category, and country of birth, every month, so you can see where you stand without decoding the chart yourself. You can see how it works on our priority date tracker page. https://www.inhousecore.com/priority-date-tracker
TLDR
Your priority date is your place in the green card line.
It's the I-130 filing date for family cases, the PERM acceptance date for most employment cases, or the I-140 filing date when no PERM is needed.
Find it on your I-797, compare it to the Visa Bulletin each month.
A priority date date generally stays with you when you change jobs.
This post is for general information only and is not legal advice. For guidance on your specific situation, please consult a licensed immigration attorney.
