write letter to your father and invite for amavasya.

Dear student,

Here is a sample letter for your reference. However, the second part of your question is not clear due to lack of information.

23,

Kothrud,

Pune-376326.

5 November, 20xx.

Dear ABC,

I hope you are doing well in your internship at Apollo Hospital.

I have recently joined Symbiosis College for pursuing MBA. As you know, I always wanted to do this post graduate course and getting admission into such a prestigious college was fortunate. I was excited as I entered the college gates. My happiness was short-lived as some seniors surrounded me. They started firing questions at me-my name, school, hobbies. I was flustered and mumbled the answers. Somebody told me to sing, another senior asked me to do rolls on the ground, yet another told me to stand like a statue. I did not know who to listen to and follow. Suddenly, they all burst out laughing and said this was not ragging but a unique welcome! They introduced themselves, offered me a cold drink and then helped me find my class-room. 

I was relieved and grateful for their help. I ended up making many friends on the very first day of college. Now, I love going to my college.

Yours lovingly,

PQR