By Mains, I am assuming you mean JEE Mains 2021. (Please do correct me if I am wrong! )

Now, the syllabus for JEE Mains 2021, it will contain most of the topics covered in class 11th and 12th in Physics, Chemistry and Mathematics. Now, one important thing to note is that CBSE has reduced the 30% syllabus for the 2021 session due to COVID-19, so it might lead to a possible slight change in the syllabus for Mains. The exact topics will be released by the official body, but you can check the exhaustive list of topics on this link-

https://learn.careers360.com/engineering/jee-main/

The exact dates have not been released yet, thought the expected dates are expected to be similar, with JEE Mains 1 happening in January, and Mains 2 happening in April. You can check additional dates on this link if you are interested further-

https://engineering.careers360.com/articles/jee-main-exam-dates

Hope this helped!