While Pind Daan can be done anytime as per family need (especially after a death), the most spiritually potent time is during:
Pitru Paksha (15 lunar days before Sarvapitri Amavasya, usually in September–October)
Gaya Mahatmya days (as prescribed in lunar calendar) Performing Pind Daan during these times ensures maximum spiritual merit (punya) and relief for the souls.