Last date for Gstr1 filing date for april month is extended to may 26th within this period return has to be filed till that time no late fee applicable. where as gstr 3b last date is 20th may late fee may be applicable on this...
If you had submitted the return then no interest otherwise 1 day interest will be levied in case you belong to above 5 crore turnover category. But don't pay it suo moto. Only pay interest when demanded by department.