When should a yarhzeit be observed for someone who died on 25 Adar Sheni?
Answer:
For one who was niftar in Adar Sheni, the yortzeit is observed in Adar Sheni.
For one who was niftar in a non leap year, there is a difference of opinion. The Shulchan Aruch rules that one should keep yortzeit in Adar Sheni. The Rama brings that the custom is to observe in the first Adar, this is the ruling for Ashkenazim. He mentions as well that there are those that are stringent to observe in both Adars.
Sources:
See Shulchan Aruch O:C 568:7, Rama and MIshna Brura ibid.
