Census Day is 21st March 2021 but you can complete the Census as soon as you have received the letter with your household access code. This can be completed via smartphone or computer alternatively if you require help to complete then call one of the Support Centre's who can also help complete over the phone or there is an option to request a paper copy.
Additional resources can be found at: https://census.gov.uk/downloadable-resources
Additional resources can be found at: https://census.gov.uk/downloadable-resources