Census returns are in the custody of Statistics Canada and the records are closed until 92 years after the taking of a census, when those records may be opened for public use and transferred to Library and Archives Canada.
On 01 June, 2013, the 1921 Canadian census will be transferred to Library and Archives Canada. LAC informs us that “only microfilm copies of this census exist and they will digitize the microfilms and make them available online as JPEG and PDF images shortly after their release date.”
More information is available here: http://thediscoverblog.com/2012/03/27/1921-census-countdown/
I would not expect this census to be available for public viewing for a few months after the June 1st date.