Question: when did arjuna used pashupatastra?
Answer: Krishna suggested Arjuna to use Pashupatastra after Abhimanyu was killed. Even though Arjuna forgot the mantra to use Pashupatastra, Lord Krishna took Arjuna to Kailash and requested Lord Shiva to grant him Pashupatastra once again. Next day in the battle Arjun killed Jayadrath with a celestial weapon.