Priority Date (PD) is the date when your green card visa numbers are available. It means that you are entitled to get your green card. It doesn’t mean you can get it right away. But it means you are in a stage were you are about to get your green card. So Priority Date is usually the date on which your application was started. In case of family based petitions Priority Date is when I-130 was filed. In the case of Employment based cases it is based upon when PERM was filed. Employment based cases were PERM is not involved, direct I-140 is involved such as National Interest Waivers, EB-1s - Priority Date is the date you file the I-140. Your Priority Date depends upon the date your case was started. That’s one thing you got to worry about.

 When you go to Visas Bulletin which is a monthly publication of the US Department of State it tells you what visa dates are expected to be curbed or in other words where the queue is for the next month. So in December they will issue predictions for January, in January they will issue predictions for February, then on top of that you have to worry about processing times. Sometimes for e.g.: in US Citizen’s cases there is no Priority Date back up, dates are immediately current for e.g.: spouse of a US Citizen, parent of a US Citizen, minor unmarried child of a US Citizen, there are no Priority Date   issues here. In these cases, you have to worry only about the processing times. So when you file the form the form takes set amount of time to process. Now a new concept has been started a couple of months ago, if you go to the USCIS website it tells you if you are in the United States this is the date when you can file your last step of the green card process. If you are outside USA this is the date, you can expect to receive further paper work from the consulate. It doesn’t mean you are going to get your green card right away but you are eligible or close to getting it.

Here is another variable. How much time is it taking at that consulate to schedule interviews. Some consulates are heavily overworked. Some consulates are relatively less worked. So that’s the fourth variable. First variable was Priority Date, second variable is processing times, third variable is final action dates which is also the part of Visa Bulletin a slightly different table, and the fourth is the local processing times.

Question: How can I calculate my sister's possible visa interview? I am confused with these changes on final action date etc. Her PD is 1 Feb, 2011 visa category F2B. She has already submitted dox to NVC. When she was waiting for the visa date she turned 21 and moved to f2b cat.1. When do you think it will be? Which table to consider? At the Visa bulletin one, I mean. 

First of all, if her Priority Date is current normally within a few months, probably less than six months. At the Visa Bulletin the final action table tells you approximately when she can expect her final documents.

Question: Once the other parent becomes citizen in USA would it be helpful to submit another petition for her? Would you advise it to be in the safe side? 

You can. You can file multiple cases for the same individual. That is not a problem. As long as you see some gain in it depending upon the country she was born in, in filing through two separate categories you can certainly do that.