Starting in 1905, a special census was taken 5 years after every national census to 1957. From 1957 onwards, the provinces were included in the national census.
This was the first census to ask about military duty, since it was taken in the middle of the First World War. The people in the military were counted as being home, and the census showed the person either being overseas by the use of the letter "O" beside his name, or if he was in Canada, the use of the letter "C" beside his name.
The people were asked the following questions by the enumerator, and they were:
name of each person in the family
the number of people within the family or institution
place of habitation
relation to head of household
country or place of birth
year on immigration to Canada
year of naturalization
can speak English/French
other language spoken as mother tongue
can read and write
Glenn Wright—a lecturer and the "behind the scenes" guy on the popular Canadian TV show, "Ancestors in the Attic"—gave a recent talk in Ottawa where he said that searching the census is the most important thing to do when starting your family tree - right after getting certificates for the births, marriages, and deaths in the family.
He said that people do not take enough time to read the census page completely enough. He encouraged people to "linger over a census page" - take a look to see when the census was taken, who were the neighbors of the people you are looking for, and how they spelled their name (is it different than how you spell(ed) your name?).
He warns that they are not a perfect record - they are a snapshot of the country on a particular day, at a particular time.
If you wish to find an ancestor in a non-census year, try looking in a city or a country directory.
A limited number of such directories are on the Internet, and it may take a bit of looking around, but they are a good resource to use if there isn't a census for you to look in for that year.
He gave the best places to look for Canadian censuses, and they are the Canadian Genealogy Centre www.collectionscanada.gc.ca, Ancestry.ca www.ancestry.ca, Automated Genealogy website www.automatedgenealogy.com, the Church of Jesus Christ of Latter-day Saints www.familysearch.org, and All Census Records at www.allcensusrecords.com.
Rumor has it that Ancestry.ca will publish every Canadian census that has been released by Statistics Canada (to date) this summer on their site.
So you can expect to see the censuses for 1851/52, 1861, 1871, 1881, 1891, 1901, 1911, as well as the 1906 Western Census, and the 1916 Prairie Census.