Do I need to know my Gotra or ancestor’s exact death date?

If you do not know your Gotra, the pandit uses Kashyap gotra — the scripturally accepted universal substitute. If you do not know the exact death date, the ceremony can be performed on Amavasya (new moon day) or during Pitrupaksha. The Mahalaya Amavasya is specifically for ancestors whose tithis are unknown.