when the registration for jee main 2021 will strt?

Registration for Jee Main 2021 examination may start from the Month of November 2020. However any specific dates have not yet announced by the authority. For furthur updates you may check the official website. You can also check this link. Once the application releases, you can apply from this link http://engineering.careers360.com/articles/jee-main-application-form
Usually Jee Main January session is conducted in the first and second week of January. Now due to this pandemic the situation is not in our hands. So till now any specific dates for Jee Main January has not yet released. And it is quite difficult to predict any date for the examination.
MHRD did announce that the syllabus of Jee Main 2021 will be reduced due to this pandemic it would be difficult to complete the entire syllabus. To complement this, CBSE has reduced the syllabus by 30% from class 9 to 12. And it is expected that this reduced syllabus by CBSE will be retained for Jee Main 2021. For the reduced syllabus you can check this link http://engineering.careers360.com/articles/will-jee-main-syllabus-be-reduced
The official syllabus for Jee Main 2021 will be released by the authority in due course of time.