Millions of census packages are arriving in the mail and one in four households will be required to complete the once-again-mandatory long-form survey.

Canadians are urged to complete the census online, using an access code they'll get in the mailed envelope, but they can also fill out a paper version.

Statistics Canada's aim is to once again capture reliable, high quality data to the same level as the 2006 census, the last time the long-form census was delivered.

It's being pitched as important to helping guide decisions on services like schools, roads, health care, policing, transit and social services.

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Besides the reintroduction of the compulsory 36-page long form – the former Conservative government made it voluntary for 2011 and the Liberals have restored it in the name of evidence-based decision making  – there are some other differences in the questions being asked.

This year's census won't ask you about your income – Statistics Canada will get that from the Canada Revenue Agency.

Nor is religion a question, as it's only asked once every 10 years.

The census now uses "sex" instead of "gender" and respondents are to answer male or female.

"Transgender, transsexual, and intersex Canadians should indicate the sex (male or female) with which they most associate themselves," it says. If they can't make that choice they can leave sex blank and explain in the comments at the bottom of the form.

Thanks Canadians for responding in such high numbers. The site was overwhelmed by the enthusiasm, but we are back!

— Statistics Canada (@StatCan_eng)

Nice to see enthusiasm for the long-form census' return. We’re committed to data-based decision making.

— Justin Trudeau (@JustinTrudeau)

Census Day is officially May 10, but Canadians are encouraged to answer the questions as soon as they receive the forms.

And they've been doing so in droves.

The volume of people logging into the census website was so high on Monday, Statistics Canada said its servers were temporarily overwhelmed.

While some Canadians took to social media to denounce the mandatory long form as invasive, many others are enthusiastic, posting .

Refusing to complete the census is against the law. Those who fail to provide information or give false answers can face fines of up to $500, up to three months in jail, or both.

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