Dear Subhankar,
For the first semester theory courses ending with code 131 or the course on Environment, please solve and submit the assignment by June 30, 2020, if you can.
If you cannot submit by June 30, 2020, then do not worry. You can solve the fresh set of assignments that will be uploaded on July 1, 2020 and remain valid until June 30, 2021.
The assignments valid from January 1, 2020 till December 31, 2020 are for the second semester courses ending with code 132 as you must have seen.
Those you can submit later: By September 30, 2020 if you wish to appear in their exams in December, 2020.