When do we light a Yahrzeit candle?

Shiva

It is a Jewish custom and tradition to light a Yahrzeit candle in the seven days following the passing of a family member, the candle must stay lit throughout the entire shiva, even on the Sabbath.
Please note! The lighting of the candle is unrelated to the customs of mourning!
Even in a case where the mourning was discontinued due to one of the holidays (for example, a man who passed on Passover eve, then Passover supersedes the shiva mourning).
The candle is still lit for 7 days from the moment of passing.

Yahrzeit

The custom of lighting on the Yahrzeit in which the passing took place, according to the Hebrew calendar.

On special occasions

On holidays: Yom Kippur, Simchat Torah, the seventh day of Passover, Shavuot, in which we observe the remembrance of souls, it is customary to light a Yahrzeit candle for parents who passed away.

The entire year of mourning

The duty of lighting is on the days of the shiva, it is important to know that there are circles in Judaism who light a Yahrzeit candle throughout the entire year of passing, and it is the custom of Chabad as well. 

Lighting a Yahrzeit candle – the location

During the days of the shiva 

The candle should be lit in the home of the deceased, preferably in their room.
If the passing took place in a hospital, it is customary to light a candle in the house where the deceased used to live. If this is not possible, the candle is lit in the mourners’ house
On the Yahrzeit
The candle can be lit in the synagogue or at home.

Yahrzeit candle prayer

There is no specific prayer said when lighting the candle.
But it is customary to say “I light the candle to bring peace to the soul of father / mother with their name.
Ashkenazi Jews: The name + the name of the father is said, Sephardi Jews: The name + the name of the mother

Why do we light a Yahrzeit candle?

Enjoyment for the soul

Our Rabbi Bahya, one of the greatest Kabbalists, buried near the holy city Safed, wrote a wonderful thing, and I suggest that you read the following lines carefully:
“It is known that the soul enjoys during the lighting of the candles and it walks with grace and happiness and spreads and expands due to the enjoyment of the light…
In other words, our Rabbi tells us that since the soul is spiritual it is drawn to the light.
Our world is material, but the most refined matter in our material reality, which could be said to come close to being spiritual, is the candle, and by lighting the candle we bring enjoyment and benefit to the soul.

Atonement

Chofetz Haim, in his book “Clear Mishna”, provided another reason – that lighting the candle constitutes atonement for the soul for which the candle is lit

Which candle is used?

It is preferable to use an oil or wax candle if there is no standard candle at home. An electric candle can be used.
In the discussion regarding electric Yahrzeit candles, there is dispute whether it can be used to bring peace to the soul