Q. What should one do if one forgets to light an Izkor candle for parents as he always lights?
A. On question 2942 regarding If someone forgot to kindle a Yohrzait candle on that day for a father or mother, what is the correct thing to do, we wrote: ‘If one forgot to kindle a Yorzait candle on that day for a father or mother, Yehuda Yaale (Y.D. 315) rules that he should donate the value of the light for tzedaka.
Horav Shlomo Miller’s Shlit’a opinion is that he should light a candle the day he remembers. Lighting the candle in the morning is also after the fact acceptable.’
However, regarding Izkor candles or Yohrtzait candles to be lit on Yom Tov itself, Poskim disagree if it is permitted or no. Kitzur Shulchan Aruch 98: 1, prohibits lighting a Yohrzait candle during Yom Tov.
Horav Aharon Miller mentioned that Horav Shlomo would be particular to also light the Izkor candles before the beginning of the first day of Yom Tov.
He quoted that nevertheless, Biur Halocho (514 d’h Ner) permits to light a Yohrzait candle in need inside the room one eats on Yom Tov as it adds more brightness and it becomes a needed light. Still, Biur Halocho recommends to light it in shul for the honor of his parents. (See also Tzitz Eliezer 6: 10, Arugas Habossem 110 and others).
Rabbi A. Bartfeld as advised by Horav Aharon Miller Shlit’a