If you know the date and time of the death of your family member then you can find Hindu Tithi prevailing at the time of the death. Annual Shraddha is performed on the same Hindu date every year. Hindu date varies every year on Gregorian calendar as it depends of the position of the Sun and the Moon. You can find the day of annual Shraddha with the help of Tithi Calculator.
Tithi Calculator also lists the suitable Shraddha day during Pitru Paksha which is also known as Mahalaya. Pitru Paksha which is a period of 15 days is the most appropriate time to perform Shraddha rituals. However Pitru Paksha Shraddha is performed only after one year of the death.