This seems a topic that has generated much discussion, so I thought I'd comment on it.
In the United Kingdom we are having a census on Sunday 27th March 2011. It can be completed online or by sending back a census form that has been delivered to most households by now. The census is being carried out by the Office for National Statistics (ONS). Their website says, under the heading "When to return the questionnaire":
Census day is Sunday 27 March. Your answers should be about your household on this day. Please submit or return your completed questionnaire before, on or after this date.
On any analysis that seems a bit odd. Surely you should only be returning your census form after the date of the census since, though you can have a pretty good guess what the answers to the many questions might be, you can't know for certain.
10.—(1) Every prescribed person to whom a household pack has been delivered or on whose behalf delivery was taken under these Regulations must, on the day after census day or as soon after as is reasonably practicable—
(a)complete the copy of questionnaire H1 included in the pack, place it in the reply-paid envelope provided and send the questionnaire to the Authority by post; or
(b)return the information requested by questionnaire H1 electronically using such an electronic system as the Authority may provide for this purpose and in accordance with the instructions included in the accompanying pack.
Which quite clearly does not allow an early census return. Failure to comply with regulation 10 is made a criminal offence (albeit a very minor one) by section 8 of the Census Act 1920. There doesn't appear to be, in the regulations or the act, any power for the ONS to disapply these provisions or vary when the census returns may be made.
I'm not a census lawyer, so maybe I'm missing something here. What's more I doubt that it would be easy to prosecute someone for doing what the ONS has precisely told them to do. It still seems to me to be an odd way to run a census.