The dates that each census was taken were:
1861 - April 7-8
1871 - April 2-3
1881 - April 4-5
1891 - April 5-6
1901 - March 31-April 1

The Census, from 1861, was organised by Registration District (these are the names in Bold in the index) followed by Registration Sub-District. Each sub-district is complete on one or more Pieces, numbered consecutively, and grouped together into RG (Registrar General) Classes, one per census year.

Within each RSD there were Enumeration Districts numbered consecutively and sufficient were allocated for each Civil Parish. Within each piece the Folios were numbered consecutively without regard for ED boundaries. Within each ED, the Schedules were numbered consecutively as issued. These schedules were distributed to households and collected after Census Night by the Enumerator. No doubt he helped some people to fill them in as well. He then took them home and copied them into his book in order. At the start of the book he described his district and at the end are some totals by page. The books went up the chain for counting and other mining for statistics during which time they received additional marks and adjustments. Over the years some pages have been lost, all in all leaving them in the state we see them now.

To reference a household uniquely, it is usually sufficient to quote either, for the Public Record Office:—
RGcc/pppp Folio ff Sched ss
where cc is the class number and pppp is the piece number

or, for the County Record Office:—
RD Name, RSD Name, ED e, Page pp, Sched sss

but it can be helpful to also add the Civil Parish as well. The Ecclesiastical Parish can be useful when correlating entries with parish records but played no part in the Census except being recorded for information. The RD and RSD are contained in the index, the transcript pages themselves contain the rest of the information.