Living is all about belonging to some tribe or community, place, culture. Culture, traditions and value systems are what makes you who you are. Ancestors give us following 3 things, which we should pass on:
1) DNA/Genes;
2) Racial, Cultural, tribal or Communal Identity; and
3)Belief and value system.

Significance of Ancestor Worship:
1) Social Stability to maintain family bonds and social order.
2) Cultural Identity provides a framework for understanding family history and relationships.
3) Psychological Comfort by providing a sense of continuity and connection to the past, offering comfort and a sense of purpose.

Everyone will like to pay respects to ancestors. How differs?

We all talk about: Body-Mind-Spirit.
Body is Physical (including brain). Mind is about Intelligence and emotion.
Spirit or Soul or life force or Consciousness is the unknown or missing link. This subject is beyond observation and experimentation, just guess work. Many believe, Soul retains personality, memory after death. They also need to eat, roam, think, and feel bad & happy, like life on earth. So, they can bless or curse.
Pithrus or spirits are beyond physical world. Heaven, Hell and Pithru loga are abstract concepts like happiness, sadness, fear and suffering. They may not be physical worlds or entities.
Are Pithrus just memory of dead ancestors (existing in the mind of living)?

Pithru Rituals

Maathru Devo Bhava; Pithru Devo Bhava; Aachaarya Devo Bhava; Athithi Devo Bhava; Worshipping these four is given highest significance and importance in Hindu Dharma. Among the four, Mathru (Mother) and Pithru (Father) Aachaarya (Guru) have attained greater prominence since they are the people who are responsible for our birth, culture and existence. Shraddha (includes Tarpan) is the ritual performed for the departed souls of ancestors, parents and relatives. It is a way of telling them that they are still an important part of the family and they still reside in our memories.

The family observes a ten or thirteen-day mourning period for dead ones. Some monthly rituals for another one year. Each year, on the particular date (as per their calendar, since there are many calendars in use) when the person had died, the family members perform annual ritual. Every month on new moon, many indian sects do Tarpan.

Fortnight of the ancestors

Mahaalaya means great destruction. There might have been a great calamity like great floods and they would have started this ritual after that for the departed souls in calamity. Mahalaya marks the end of the fortnight-long Tarpan to the ancestors. This is similar to day of the dead by other cultures like: Gai Jatra of Nepal; Galungan in Bali; Pchum Ben of Cambodia; Chinese Hungry Ghost Festival; Dia de los Muertos of Aztec/Mexico; Ari Muyang of Mah Meri tribe in Malaysia; Chuseok harvest/ancestor festival for Korean people; Obon of Japan; Guy Fawkes Night/Halloween; and All Saints' Day/All Souls' Day. While in essence, all remains same; procedure, customs and rituals may differ from region to region.

Some notes from scriptures

Garuda Puraana”, deals at a great length on life after death. After death, the soul in its disembodied form hovers about its original and familiar places for ten days. It starts on its journey to the judgement seat of Lord Yama, the God of death.

Souls move, think, and feel like human beings. Their one day is equivalent to our one-year time. That is why anniversary (in our terms) is taken as day for Pithru karyas. Some wise men who have traveled towards polar regions has described tough journeys. So, based on these travle notes, came the idea that souls go to world (like polar region), where one day is one year, six months day and six months night.

Different cultures have different routes and time line to reach the God of death. For example Maori of Newzealand believe souls travel through the beach and go to northern most point of the island.

Some thoughts on Rituals

Let us keep these controversies aside. These rituals create some meeting opportunities. Some make their living, by conducting these rituals. They also serve as some social event. Let us pay our respects to ancestors, to whom we owe our existence! Let us benefit from good points on these rituals.

Notes on traditions around the world

• Egyptian/mesopotomia folk stories discusses how souls after life, have to travel towards God of death for Judgement and then go to heaven/hell.
• Ancestor reverence was popular in Greek and Roman homes in antiquity.
• One can see “religion of the home,” or rituals centered around the family’s ancestors in Japan. Japanese ancestor rituals, primarily centered around the Obon festival and butsudan altars, involve honoring and acknowledging the spirits of deceased family members. Obon Festival (usually in July or August) to welcome back the spirits of ancestors visiting their homes during Obon and can offer blessings or guidance. Butsudan Altars are decorated with images of deceased family members, used for Praying, chanting and offerings like food, drinks, and flowers. The ancestors are believed to be in a higher state of existence and are revered as hotoke (deity). Offerings are a way to show respect.
• Ireland is considered the birthplace of modern Halloween, with its origins stemming from ancient Celtic and Pagan rituals and a festival called Samhain, or Samhuinn (end of the light half of the year).
• All Saints' Day, November 1, is known as Ognissanti, the festivities usually begin a couple of days before, when people begin leaving fresh flowers—generally chrysanthemums—on the graves of departed loved ones, as well as complete strangers, turning the country's cemeteries into a beautiful display of colors. On November 1, many Catholics around the world celebrate All Saints' Day, followed by All Souls' Day on November 2.
• Germany has its own tradition: Many hide their kitchen knives, so that returning spirits won't be accidentally harmed (or use the same knives to harm the living). Halloween at Vlad “The Impaler” Tepes’s purported home at Bran Castle in Transylvania, Romania. In early November, people across Poland travel to cemeteries to visit the graves of their family members. Dzień Zaduszny is like the equivalent of All Souls' Day.
• From November 1 to November 2, Mexico and parts of Latin America celebrate Día de los Muertos (Day of the Dead) to honor those who have passed away.
• Pangangaluluwa is a tradition in the Philippines in which children go door to door, often in costumes, where they sing and ask for prayers for those stuck in purgatory.
• On the 15th day of the seventh lunar month, which is around mid-August to mid-September, the Chinese celebrate the Hungry Ghost Festival.
• The Awuru Odo Festival marks the return of dearly departed friends and family members back to the living. From the end of September to the middle of October, Buddhist families gather together to celebrate Pchum Ben, a religious holiday to celebrate the dead.

Riddle: Death and after Death

1) உடல் கரைந்தது. மனம் கலந்தது. உயிர் மறைந்தது
Body dissolves into natural elements. Mind (Mental impressions, experience, memory etc) can get mixed with information sink of infinite capacity. Alive in minds of living for a long time. Life disappears (into nowhere?)
After some one is dead, we are not able to erase dead from memory (One can not). Contributions of dead (positive or negative) is still there. Past interactions with them haunts us. Our past community and religious teachings, society beliefs and culture, influence our relations with dead. So, Living needs rituals and other formalities for this.
Are Ghosts/pithrus mental objects, mind sees (like in a dream)?

2) Some wise sages have handled after death question very well.
இறப்புக்கு பின்னர் என்ன ?
கண்டவர் விண்டிலர் - விண்டவர் கண்டிலர்.
Dead has not come back to say, what happens after death. (Some believe in returning after temporary death or near death experiences. Still it is not real death, point of no return).

3) What to do about death?
You are not going to be there. Why bother or think about it.
We can experience life. Death refers to absense of life. Death is similar to darkness, which is merely the absence of light. Should we think about life than death. Easy to say, but very difficult to avoid thinking about death. Fear of unknown (what is after death?).

4) What rituals to do for dead person? What does dead want?
One should ask the dead, what it wants?
Rituals are for people left behind. They can tell what they want to be done for their personal satisfaction.
Dead does not need or ask for anything. It has left everything behind.

5) உள்ளிலிருந்து ஆட்டுவது சிவம்
உணராது ஆடுவது சவம்
சவத்தை விட்டது சிவம்
கதையை முடித்தது சிவம்
Siva residing inside, makes one to dance/move.
We (bodies) may dance with out recognizing siva inside.
Once siva leaves, no movement, body left is a corpse
Thus siva ends one's story.

6) உண்டது கடன். விட்டது கடன். (சிவம்) கணக்கு நேர்
What you consumed or ate is debt. Debt paid when dead leaves everything. Account is settled.

7) காயமே இது பொய்யடா! (இது)
காற்றடைத்த பையடா! கிழிந்த பையடா!
கிழிந்த பையே மெய்யடா! பையைத் தையடா!
Body is false. Just air filled delicate bag which can be torn
But Body was called mei, which also mean Truth or what we experience
Life is repairing/mending the body and taking care of it.

8) The Japanese Emperor asked the Zen Master, Bosho: "What happens to a man of enlightenment after death?"
Bosho replied: "How should I know?"
The Emperor continued: "But you are a Master!"
Bosho replied: "Yes, Sir. But I am not dead yet."
"NO ONE KNOWS" is the correct and safe answer.
If you want to know death, you can only know it by experience. You know life by experiencing it. If you try to understand things which you do not experience like death, you can come out with theories or opinion or belief, which can neither be proven or rejected.
Humans can not accept, there are things, Unknown or beyond our Knowledge/ability.

9) Creation and destruction are the two sides of the same reality. Creation begins with destruction and in creation there is already a seed of the destruction to come. If the birth of an individual is creation, his death is destruction. Ancient philosophers (especially from east) view life and death as a continuum, believing that consciousness (the spirit) continues after death and may be reborn. Everything is just the cycle of life, death and rebirth.

There is no standard answer from different scriptures on soul or what happens to soul after death.
Beliefs on how long a soul stays after death vary wildly, ranging from immediate departure to a 40-day lingering period. Religions and spiritual traditions, suggest different transition period. Can soul departure, depends on faith followed by soul?