Every year shradh is performed on his/her death's date as per the Hindu calendar. If you don't know the date but a month is known then on the moonless night (Amavasya) of that particular month this pitru dosh puja is performed.
Yes, Pitra Dosh does exist. people have several misunderstandings about it in their minds. It is formed by different planetary combinations and placements in a horoscope. Pitra Dosh generally has serious and long term effects on a person’s life.
“Om Shreem Sarva Pitra Dosha Nivaranay Klesham Han Han Sukh Shantim Devi Phat Swaha” is the mantra.
-Use only Cow Milk, Crud and ghee.
-Use Mustard, barley, peas, and Kangana.
-Use gold, silver, copper for rituals
-Don't eat Non-vegetarian food
-Avoid constructing a new house