It comes only once a decade and today is the day the British public will answer a series of questions about themselves in the national census - or risk a £1,000 if you don't.
The census has gathered information on every person and household in the country since it was first introduced in 1801.
Today, every household in England and Wales - although not Scotland, which has postponed its until next year due to the pandemic - will complete the form online for the first time.
So, what is the census and why is it so important to complete it today that you could be fined for not doing so?
We do our best to answer some of your questions below...
What is the census?
It takes place every 10 years and in an official count of every person and household in the country. The point of it is to give the government an accurate estimate of who lives where and what they do.
The final results should provide "a detailed snapshot of our society". This helps inform those in power where and how to allocate funding for local services such as education, doctors' surgeries and roads.
The nationwide survey is run by the Office for National Statistics (ONS).
When does it take place?
Today - for everyone living in England, Wales in Northern Ireland.
Scotland has delayed its version of it until March 2022 because of the Covid pandemic.
But the good news is that only one person in each household needs to fill it in - either online or the physical form.
What will it ask me?
The questions ask every person living at the property about their age, race, occupation and relationship status.
Some of the questions, such as those about gender identity, are voluntary and people can request a separate form to fill in to protect their privacy from other people in their household.
Will I get fined if I don't complete it?
Money Saving Expert founder and consumer champion Mr Lewis said: "You can be fined up to £1,000 if you don't complete the census.
"It's an offence to supply false information or not to complete the census, and you could be fined up to £1,000.
"Fines are a last resort though.
"In England and Wales, four people received the maximum £1,000 fine for non-compliance at the last census in 2011, while a further 270 people were fined an average of £218."
So, probably best to make sure you get it completed today. If prosecuted, you may have to pay a fine of up to £1,000 plus court costs.
Providing false information can also lead to a fine.
Should I complete it online or on the paper form?
Although a paper form is sent out to all households, people are encouraged to fill it in online this year.
Visit census.gov.uk to find out more.
When will the results be released?
The full details are kept under lock and key for 100 years - so, unless you are very lucky in life, it will most likely be our ancestors who reap the benefits of the full census.
But initial findings will be published in March next year, while another release comes another 12 months later in March 2023.
That data will allow us to access information at local authority level.
