There is a broad discussion/debate in the Gemara and Poskim which of the two months of Adar is the primary one, while the other is merely an addition. Is the regular Adar the first one and the second Adar is the addition, and, therefore it should be called Adar Sheini? Or, perhaps the primary Adar is the second one because it completes the year and the first Adar is the additional month of the leap year?
The Mishna and the Gemara discuss this issue in the Tractate of Nedarim 63a. The braita brings a tanaitic dispute how to write the date in contracts: according to the opinion of Rebbe Meir in the first Adar one writes Adar Rishon, and for the second Adar one writes just Adar. However, the opinion of Rebbe Yehuda is just the opposite – for the first Adar one writes just Adar, and for the second Adar one writes Adar Sheini.
In Megilla 6b the Mishna states: ” There is no difference between the first and second Adar except for the reading of the megilla and gifts for the poor.” The Gemara expounds this idea further. In this context we will not discuss the halachic dimensions, but, we do want to make one interesting comment.
This question has practical halachic ramifications that are mentioned in all four sections of the Shulchan Aruch:
Orach Chayim 54:10 – determination of the date of a bar mitzvah.
Orach Chayim 568:7 – determination of the yarzheit and fasting on it.
Orach Chayim 685 & 697 – the laws of the arba parshiot and Purim.
Yoreh Deah 220:8 – the law regarding one who took an oath until Adar.
Even HaEzer 126:6 – the correct writing of the date in a get.
Choshen Mishpat 43:28 – the law governing the sequencing of contracts.
David HaMelech said, “?÷??? ?´ ????? ????? ??. – The precepts of G-d are straight and they rejoice the heart.” If so, we have a new perspective to the saying, “?????? ??? ????? ????? – When Adar begins we increase our joy!”