Note: This is not legal advice, it is intended for educational use only if you have a legal issue consult competent counsel.
The priority date is a methodology used by the Department of State (DOS) to manage the issuance of visa numbers. The Congress has limited the number of visas available each year using a complex system of quotas for the various immigration categories and origin countries. A discussion of the quota system is beyond the scope of this article.
The DOS publishes a monthly visa bulletin which lists each category and some specific countries that normally are qouta limited (most are) and what the most recent current priority date is for that month. If an immigrant's priority date is current then they are eligible to receive an immigrant visa. If they are in the U.S. their adjustment of status application can be filed and/or adjudicated.
A priority date is current if it is equal to or older than the published date.
Your priority date should be listed on the receipt notice for your immigrant petition - either I-130 (family) or I-140 (employment). However, the absence of a priority date on that receipt has no meaning. It is simply missing and you need to find your priority date through a more in depth review.
Unfortunately no. I republish the monthly State Department Visa Bulletin on my website. The Visa Bulletin can and will show movement in both directions from month to month. This is why your attorney should have a system to track your priority dates and should receive alerts when your date is current. Also note that priority dates move forward and backward (called retrogression) in unpredictable increments. It is not uncommon to find the movement exceeding the actual passage of time from one month to the next For example, the October 2008 bulletin show several categories moving forward by many months and several categories moving backwards by many months from the dates in the September, 2008 bulletin.