The Jewish mourning period is an entire year (12 lunar months), even though Kaddish is recited for only 11 lunar months minus one day. Tradition has it that all souls are usually sent to Purgatory for the first year after death. The recitation of the Kaddish is supposed to raise the soul one level at a time (monthly), from Purgatory to "Gan Eden" where the rightous dwell. Those who are deemed worthy are given a month off for good behavior during their lifetime. As we humans are not the proper ones to judge past behavior (only God can do it), we must assume that each and every soul merits only 11 months in Purgatory and not the full 12. That is why Kaddish is only recited for 11 months, but the mourning period must be observed for an entire year until the Yahrzeit. That day is determined as the anniversary of the day of death by the Jewish (lunar) calendar. Kaddish is then recited on that day and a 24 hour candle is lit as well, each and every year thereafter. By the way, the mourning period of 12 months is only for parents. For all other relatives (spouses, siblings, and children), the mourning period is only 30 days. SOL ZELLER