My dad won't listen to me. What should i do?

There's kind of a family reunion out of town this coming Friday and he's asking me to take off my internship on both Friday and Monday to attend it. Problem is its a paid internship at the Pentagon with the DoD, so its not some bs firm or company, and even worse its my first week. I do not want to make a bad impression on the first week and ask to take days off. On top of this it is a paid internship and i will not be paid for those days. I already told my dad that I cannot go and I do not want to take time off of work and hes like "let me call your supervisor," like it was some sort of freakin parent teacher conference. How can i let him know i am not interested in going without him throwing a hissy fit and almost forcing me to go?