Prostrating on a grave three days after the anniversary of death
Question
What is the source for the idea that three days after the anniversary of death, it is still considered the day of the anniversary?
Answer
There are books that mention this [Birkat Shlomo - Keter Torah (Radomsk, end of portion Truma), Sefer Nachala LeYisrael (Hibner, question 51)].
And there are those who have proven, from the book Pri Etz Chaim (Counting of the Omer, Chapter 87), that the AriZal stayed in Meron for three days when he came to perform the first haircut for his son.
However, in practice, this matter is not agreed upon. See the book Devar Torah (by the author of Minchat Elazar, Talitha'i, Chapter 46) who rejected this proof. He also mentions that his teacher, the author of Darkei Teshuva, did not hold that there is a point in prostrating during the three subsequent days after the anniversary of death.
Source
See in the body of the response.