Priority dates determine the waiting period for a visa number in the family preference categories that have annual numerical limits. 
A petition that is properly filed by the Immigration and Naturalization Service (INS) receives a date of approval. This date is called the priority date. 
The priority date controls the order in which the beneficiary will receive a family preference visa depending on the waiting period in each of the four family preference categories. 
Some times when a petition is already filed and a priority date is given, there may be certain circumstances that may affect the priority date or the waiting time for the beneficiary to receive a visa number. Conditions such as marriage, divorce, turning 21, naturalization of the petitioner or death can affect the waiting period for the beneficiary.

Examples of change circumstances:
Marriage: If for instance a U.S. citizen was filing for her 19-year-old daughter, she would have been considered an immediate relative and at the time the petition was filed she would have received a priority date according to the category, making a visa immediately available to her. If however, the child got married before a visa number becomes available, her category will change to a third family preference and she will be considered not a child but a married daughter. In this case the priority date remains unchanged but the waiting time for a visa number may be much longer. If however, the petitioner was only a lawful permanent resident the beneficiary will be totally disqualified. A parent must be a citizen to file a petition on behalf of a married son or daughter.

Divorce: An approved third family preference petition, that is a married son or daughter who is 21 years or older can be converted to a first family preference, unmarried son and daughter if the beneficiary divorces or otherwise legally terminate the marriage. 
If the married son or daughter was under 21, he or she will be converted to an immediate relative.
In both cases the petition is approved on the date of the divorce.

      Turning 21: Recently Congress passed a law that states if a child turns 21 after his or her petition has been approved the child will still be considered an immediate relative and will retain priority date. Before the law was passed in August, a beneficiary who turned 21 or "aged out" before receiving a visa number would be converted to another preference. A derivative beneficiary is not eligible; only a direct beneficiary can benefit from this law. 

            Naturalization: An approved second preference petition converts to an immediate relative for a spouse or an unmarried child as soon as the petitioner gets naturalized. The petition will be deemed approved from the date of the petitioner's naturalization. The priority date remains the same, but the conversion to the immediate relative shortens the waiting period for a visa number to become available. 

        Death: The death of a petitioner or beneficiary will revoke any petition that was filed. However there may be cases when the petitioner dies and the attorney general will determine that the petition should not be revoked for humanitarian reasons. 

         When a petitioner claims an early date from a previous petition, it is important to inform the U.S. consulate, the National Visa Center and the INS of that date. Those agencies should also be notified of a change of circumstance of the petitioner or beneficiary. Necessary documents like divorce decree or naturalization certificate should be accompanied with the notification.