"The custom of saying kaddish for the iluy Neshama of the deceased parents is has very ancient roots. It is brought down by Chazal that this has great benefit for the soul of the deceased, and even for a complete tzaddik – to bring him up, step by step, in Gan Eden. A such, it is important to say kaddish also on Shabbat and on Rosh Chodesh. Even more than regular kaddish – kaddish which is said after learning Torah and after hearing a derashah surely bring great nachat to the deceased. Saying kaddish is also part of the obligation of respecting one's parents [throughout the year] after their passing, and also on their yahrtzeit every year. This is included in the obligation of kibbud av va'em, as we said above – "One must respect them in their life and after their death". How does one show his father respect after his passing? By intensifying his father's honor in all possible ways – for example, by referring to him as "Abba Mori" and by giving him nachat ru'ach after his passing, by saying kaddish, lighting a yahrtzeit candle, and making an azkarah on the yahrtzeit, getting an Aliyah to the Torah on the yahrtzeit, etc."