Yartziet in Adar
Does one commemorate an Adar yartziet in the first Adar or second?
According to Shulchan Aruch O:C 568:7 the yortzeit is commemorated in Adar Sheni as with all other mitzvos of the month of Adar. This is assuming the year of the petirah was a shana pshuta. If the petirah was in Adar Rishon, the yortzeit should be thus commemorated as well. In addition the Bais Yosef [Y:D 403] stipulates that if the shana meuberes is the first yortzeit, it should be commemorated in Adar Rishon, being the end of the 12 month period after petirah. This is the custom of Sfardim.
For Ashkenazim, the Rama [ibid] holds that the yortzeit should be kept in Adar Rishon, and for one who wishes to be stringent it should be kept in both Adar Rishon and Sheni. [see m”b ibid s.k. 42 that one who actually accepted a neder to fast on the yortzeit, would be obligated to do so in both Adars as opposed to one who does so because of minhag].
As to the discrepancy between Yortzeit and other mitzvos according to the Rama, please refer to the post titled:
//en.tvunah.org/2014/01/13/bar-mitzvah-and-yortzeit-in-adar-rishonsheni/ for elaboration.
In addition please refer to //www.torahbase.org/?p=3954 which is the Rav shlita’s latest shiur on the subject of Adar Rishon and Adar Sheni.