Census organisers are urging 26 million households in England and Wales not to forget to fill in their questionnaires.
Forms should be filled in today - Census Day - or as soon as possible afterwards, the Office for National Statistics (ONS) said.
Electronic tracking systems are being used to monitor who has filled in the forms with collectors knocking on doors from April 6 where households have not yet returned their questionnaires.
The ONS said help and advice would be offered to people having difficulty filling in the form and collectors would also supply replacement questionnaires where forms have been lost or damaged.
Glen Watson, 2011 Census director said: "Completing the census form promptly and sending it back to us means no one will have to knock on your door to remind you.
"Census statistics enable the authorities in England and Wales to plan properly for the future for school places, housing, roads, emergency services and a host of other local services."
Householders are being asked a range of questions covering areas such as national identity, ethnic group, educational qualifications, job titles, travel-to-work method and state of health.
The census also includes questions for the first time on civil partnerships, second homes and recent migration.
People born outside the UK are being asked when they most recently arrived in the country, with those who arrived in the past year asked how long they intend to stay.
There is also a question asking people whether English is their main language and how well they can speak it.
There are no questions on income, sexual orientation or nature of disability.
Organisers said the information from the once-a-decade exercise will be used by health authorities, councils and a range of other bodies to plan services.
The ONS insisted that the information it gathers will be confidential for 100 years and not shared with other agencies such as the police or tax authorities.
The 2011 census has used a "much improved" address register, the ONS also said, compared to 2001 when some local authorities accused it of significantly under-counting their populations.
Critics have attacked the £482 million exercise - the biggest in its more than 200-year history - as outdated, unnecessary and an invasion of privacy. There have also been fears expressed about data security.
In 2001 - the first time a voluntary question was asked about faith - almost 400,000 people claimed their religion was "Jedi". This was in addition to almost 7,000 who said they were witches.
Daniel Hamilton, director of Big Brother Watch, the civil liberties group opposed to the census, said: "Everyone knows the census is a monumental waste of time and money.
"A large number of the questions duplicate data already held by the authorities on databases such as the electoral register, school records, tax returns and GP information.
"Last time, 390,000 people declared their religion as Jedi, 7,000 people said they were witches. There's no reason to think people will take the census any more seriously this year."
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