A member of my wife’s extended family was nifta just over a year ago. The lived alone and was ‘found’. No one knows exactly the date of her passing (either 3rd or 4th Kislev). She was, unfortunately cremated, around 5th or 6th of Kislev.
None of her direct relations are frum and we would like to say kaddish on her Yartzeit. Which date should we observe as the Yartzeit?
If possible it is better to observe both, the 3rd and the 4th by lighting a candle and say kaddish on both days, but if you can’t then the yahrtzeit should be observed on the 4th of Kislev. The reason being is that we observe the yahrtzeit on the day of the death, (although there is controversy regarding the first year, in your case it will be observed on the day of the petira since you are not the aveilim). Regarding, which day we are to consider the day of that we assume that the person that was living continued to live until we know that she died, therefore the later date should be observed, which is the 4th of Kislev.
Daas Sofrim 67-5, 36.