Whenever I sit for cutting my nails at night my dad forbids me to do so. He tells me to do it in the morning. Is it Haram in Islam to cut nails at night?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is lawful to cut nails at night as it is lawful to cut them during daytime.  Believing that cutting the nails at night is unlawful or could cause harm is nothing but superstition which Muslim should avoid completely.  So, you should point that out to your father but with wisdom and fact.

Allah knows best.