Jewish Law requires that the Kaddish be recited during the first eleven months following the death of a loved one by the mourners. After that it is recited on the Yahrzeit- anniversary of the passing.
Interestingly, there is no reference to death in the Mourner's Prayer and the theme of Kaddish is the Greatness of God. Its purpose is to reaffirm our faith at a time when we are especially vulnerable to turning away from God. We are taught that with faith we can be strong enough to endure the death of a loved one.
We are asked to view the passing of our beloved from the perspective that his/her soul was gathered in, so to speak, by the One who originally created it. After death, the soul’s spiritual level, its closeness to G-d, is determined by merit gained through mitzvot (commandments and good deeds) performed during life. Upon passing from the physical world, the soul seeks to rise closer to G-d, yet has no further ability on its own to improve its spiritual level. The living can help the souls of the departed rise closer to G-d by doing mitzvot in their memory. Traditionally, the most common mitzvot done on behalf of the departed are giving charity and the recital of the mourner's Kaddish.
In this prayer, we pray for peace - peace between nations, peace between individuals, and peace of mind. Some may choose to have Torah scholars recite the Kaddish in honor of the deceased.
A video to help you learn how to recite the Mourner's Kaddish prayer
Hadassah- Perpetual Yahrzeit Program
The Kaddish Foundation