UK Census returns were taken as follows

1801, Tuesday, 10 March. No longer exists, with a few exceptions
1811, Monday, 27 May. No longer exists, with a few exceptions
1821 Monday, 28 May. No longer exists, with a few exceptions
1831, Monday, 30 May. No longer exists, with a few exceptions
1841, Sunday, 6 June. Ages were apparently rounded down in multiples of five years, this was not done consistently but where this has been implemented, relatives aged 26 and 29, will both be shown as aged 25.
1851, Sunday, 30 March
1861, Sunday, 7 April
1871, Sunday, 2 April
1881, Sunday, 3 April
1891, Sunday, 5 April
1901, Sunday, 31 March (made available 2002)
1911, Sunday, 2 April 100 year closure was retrospectively applied, this has now been published.
1921, 19 June 100 year closure was retrospectively applied. (scheduled publication date 1 January 2022)
1931, 26 April Destroyed by fire during WW2. (in 1941) This was not caused by enemy action and appears to have been an accident. The Scottish Census for this year survived as it was stored separately, in Edinburgh.
1939 – Friday, 29 September World War II This was National Registration and enabled the issue of identity cards. (Published 2 Nov 2015)
1941 not taken due to WW2
1951 8 April (scheduled publication date 1 January 2052)
1961 – 23 April (scheduled publication date 1 January 2062)
1966  Though usually thought of a being taken every 10 years, the Census Act of 1920 makes provision for a census to take place at intervals of five years or more. A mini-census using a ten percent sample of the population was conducted on 24 April 1966. (scheduled publication date 1 January 2067)
1971 – 25 April (scheduled publication date 1 January 2072)
1981 – 5 April (scheduled publication date 1 January 2082)
1991 – 21 April (scheduled publication date 1 January 2092)
2001 – 29 April (scheduled publication date 1 January 2102)
2011 – 27 March (scheduled publication date 1 January 2112)

Notes
In the case certainly of the Manx fishing fleets and perhaps others, the census on land and on ship has taken place on different days on at least two occasions that I know of, possibly more. This provides an opportunity for a fortunate few, to tie their relatives to different family groupings. It may also be the case with other ports and in other years and the possibility is worth thinking about during research.

Census information and questions

● what do census numbers mean ?
Basically it is an indexing system, so if you have the RG, piece, folio and page then you, or someone else, will always be able to use the same details to find the same page.

RG refers to year
1841 and 1851 is HO107,
1861 is RG9,
1871 is RG10,
1881 is RG11,
1891 is RG12,
1901 is RG13

PIECE
After the RG, the next part is a census piece. Each enumerator has a pre-printed booklet to fill in. This was around 20 sheets (40 sides). Once filled in these booklets were bound together to make a book of perhaps 200 sheets representing a district. This book is the piece. It thus forms a collection of enumerators’ books for a particular district. This can contain books for several census enumerators, covering a certain predefined area.
FOLIO
Sheets in a piece were marked with a folio number on the top right corner of each sheet on the front (not the back) The folio is one sheet within the book, this is not a page (which is one side) but a sheet which is two.
PAGE
In a perfect world each folio would always consist of two sides -pages one and two, but errors mean that sometimes a folio number was missed, messed, overran, fell on the floor, got found later, got eaten by the dog, or just, well something human happened! so sometimes a folio will have three or four pages, or have had something added to it, like an annotation.

SPELLINGS

You may find various spellings of a relative’s names, especially in the early census returns. Bear in mind that that were filled in on the doorstep by the enumerator, from questions answered by your relative (often because the inhabitant was illiterate). Illiteracy also means that there was no way for an illiterate head-of-household to check or verify an entry or a spelling. If your English relative was from another area, such as Ireland or Scotland then an accent may also be thrown into the mix. Irrelevant? on census returns, I have a total of 17 different spellings for one family of my Irish relatives, some minor, some significant, yet these are all the same family, in some cases the same person appears under fourdifferent spellings.