My mother past the way last August 21 2004. It was Mיxico City 8:15 p.m. Shkia was at 8:00 p.m. Tzet Hakojabim 8:35 p.m End Shabat at: 8:42 p.m When is the exactly date of Yoir Tzait?

The Shabbat was Shoftim, Elul the 4th 5764 and Motzei Shabbat was the 5th. The Poskim dispute about a person that died after sun set before the stars appear. The Gesher Hachaim (A 32, 12) recommends keeping both days Yahrtzite if you are the only mourner and if there are mourners to keep both days you should keep the earlier day (4th) Rav Mordechai Eliyahu (Tzror Hachaim 161) says to keep the earlier day. Rav Moshe Feinstein (YD 3 159) recommends the later day (5th) May Hashem comfort you, amongst the mourners of Zion and Jerusalem