From the letter ' All households should complete the census on Sunday 21st march 2021 or as soon as possible after'
I've accessed my census on line using the code and one of the questions clearly states 'we are counting people in this household (address) on Sunday 21st March.
I didn't take it any further so don't know if it would be accepted before 21st march, but its pretty pointless to me filling in a form, which is a legal obligation, stating who was at your house three weeks in the future.