Yahrzeit is observed on each anniversary of the day of death according to the Hebrew Calendar.

One who is not certain of the day when a relative died should select an appropriate date on which to observe yahrzeit each year.
A candle should burn in the home during the twenty-four hour period of yahrzeit, sunset to sunset, starting on the evening preceding the day. When the yahrzeit coincides with Shabbat or a Festival, the yahrzeit candle should be lit before the candle-lighting for the day.
It is appropriate to fulfill some mitzvah in honor of the dead on this day. This could consist of study, leading services in the synagogue, or contributing to some worthy cause in memory of the deceased. It is also appropriate for family and friends to gather on the yahrzeit for the purpose of recalling various aspects of and events in the life of the dead, perpetuating his or her memory in a warm and intimate atmosphere.

Mourner’s kaddish is recited at all services on the yahrzeit, from evening services on the night before through afternoon services on the day itself.