The census, run by the Office for National Statistics (ONS), is a once-in-a-decade survey that gives the most accurate estimate of all the people and households in England and Wales.
It has been carried out every ten years since 1801, with the exception of 1941.
This year is the first time that the census will be run predominantly online, with unique access codes being sent to households, allowing them to complete the questionnaire on their computers, phones or tablets.
Census day is on March 21, but residents can fill their form in online as soon as an access code is received in the post.
The census includes questions about your sex, age, work, health, education, household size and ethnicity.
And, for the first time, there will be a question asking people whether they have served in the armed forces, as well as voluntary questions for those aged 16 and over, on sexual orientation and gender identity.
It is a legal requirement to complete the census. Without the information you share, it would be more difficult to understand the needs of local communities and to plan and fund public services.
The Leader of Calderdale Council, Councillor Tim Swift, said: "“The information provided by the census gives a snapshot of what life is like for people living in Calderdale in 2021.
“Having accurate data about our borough's population is very important and affects the amount of money the Council receives from central government. It also helps inform what services are needed where.
“Census data is also regularly used to research family trees and allows historians to discover more about the social history of our country. By completing your census you’re writing yourself and your family into history for future generations to discover.
“It’s important that everyone completes the census. Your details are protected by law and information published is anonymous.”
Local census support centres will be open while the census is happening.
Staff will offer help for people who aren’t confident online, who don’t have a computer or that need help completing on paper.