calendar - Why is Purim on Adar II during a leap year?

On a leap year, when a second Adar is added, we celebrate Purim on the second Adar. I would think it should be celebrated on the first Adar, since the Megillah goes out of its way in several places to identify Adar as the "twelfth month." Adar Sheni would seem to be the thirteenth month counting from Nisan.

So what is the reason that Purim was instituted to be observed on Adar Sheni during a leap year?

Answer

The gemara in Megillah 6b-7a explains why when it investigates the Mishnaic statement that in a leap year, one is required to read the megillah in the second Adar to be yotzei the requirement. (as per dafyomi.co.il)

1. R. Eliezer says, every year we celebrate in the month next to Shevat.


2. R. Shimon says, every year we celebrate in the month adjacent to Nisan.


3. We understand R. Eliezer's reason - we do not pass over Mitzvos.


4. Question: What is R. Shimon's reason?


5. Answer #1 (R. Tavi): It is better to put redemptions close to each other.



6. Answer #2 (R. Elazar): He learns from "Lekayem Es Igeres ha'Purim ha'Zos ha'Shenis" (a) Had it said only "In every year and year", one might have thought that we always read in the month next to Shevat; (b) Had it said only "ha'Shenis", one might have thought that this applied only in the first two years (after the miracle). (c) R. Eliezer expounds "ha'Shenis" like Rav Shmuel bar Yehudah.


7. (Rav Shmuel bar Yehudah): At first, Purim was enacted in Shushan; later, it was enacted everywhere.