is the OBC certificate need to be of 1st April 2021 during the time of mains application form. or it is necessary later only(councelling)?
No there is no any necessity that OBC certificate should be of date after April 1st 2021 instead it should be valid and as the application process for April attempt of JEE-Main 2021 has bee closed so now you can apply for May attempt when the application process for same will be started and suppose if you don't have your valid OBC right now then you can submit the self declaration/undertaking which declares that you will submit the original and valid caste certificate later on at the time of admission.
You can download the undertaking from the official portal of NTA, for which link is also given below :-
When you visit on the above link you will find a PDF and on page no.9 there is undertaking format which you have to download and after filling it you have to upload it on the place of category certificate in the JEE-Main 2021 Application form.
For complete details about JEE-Main 2021 Application form kindly check out the link given below :-
I hope this information helps you.