"On the first week" or"in the first week"

[moonlike]

Hi
Could you help me with these combinations of ordinal numbers and a noun after that? I'm kind of confused.
1. On the first day/ at the first day/ in the first day
2. On the first night/ at the first night/ in the first night.

Thanks a lot.

 

[5jj]

All of them are possible in the right context.

On the first day of Chrismas, my true love brought to me ...
We never do much in/on the first day of term.
I wasn't present at/on the first day of the trial.