The exact dates have not been notified yet.However Banaras Hindu University will release the BHU UET 2021 application form in February, 2021. The last date for submission and form fill ups have not been notified yet by the Benaras Hindu University.
You can check more such details in the link below: https://university.careers360.com/articles/bhu-uet-application-form#:~:text=BHU%20UET%20Application%20Form%202021%20%2D%20Banaras%20Hindu%20University%20will%20release,by%20the%20Banaras%20Hindu%20University.
Hope it helps!!