I can answer that question from personal experience, I have been to that ceremony several times.
There are no tickets or registration, It's free. (May have to pay a nominal car parking fee)
There is no NOC to obtain, however it is a sensitive area and for foreigners it will be really advisable to carry evidence of the legality of their stay at that time.
It is famously hard to prove a negative, and China is not going to issue an official statement stating that you haven't been there, so you'll need to work with what you have.
In general, from what I've seen, the vast majority of immigration checks on where you've been rely almost entirely on honest self-declaration, backed up with the force of the law if ...
Most likely this would be Kumbh Mela, which in 2013 attracted (depending on which figures you believe) up to 120 million people, including 30 million on a single day.
The full gathering occurs every 12 years, with a smaller (at least in theory) gathering after 6 years.