see 'Discover more about these records' and 'Who could vote and when' for a good summary
https://search.findmypast.co.uk/search-world-Records/england-and-wales-electoral-registers-1832-1932There were two key dates the "qualification date" which was the date when the voter had to qualify to be added to the register and the "effective date", the date when the register came into effect for any elections from that date on.
For Family History it is the Qualifying date we desire rather than the in force date (which they tend to be listed under online on Ancestry and Findmypast, in fact in Yorkshire Ancestry has some indexed under the expiry date). As that Qualifying is perhaps 4 months prior to the publication it often explains why the dates dont quite fit with deaths and marriages or 1939 Register.
Post 1945 registers tend to have the qualifying date at the start of the Polling District if page back or use the filmstrip on Ancestry, but for earlier ones see the British Library guide and Dates of registers on page 20
https://www.bl.uk/collection-guides/britishlibrary/~/media/35891524fbb546c9ac705f8e754e39cf.ashx summarised below:
• In England and Wales the qualifying date was 31 July until 1878 and 15 July from 1879 until 1914.
• From 1918 it was 15 January for the Spring register while they lasted and 15 July for the Autumn or annual registers until 1928.
• It was 1 December 1928 for the 1929 register and 1 June for the 1930-39 [and 30th June for the 1945-49 registers].
• The qualifying date for the May 1945 register was 1 January and for the March 1946 one was the preceding 1 December.
• From 1950 to 1954 it was the previous 20 November
• From 1955 to 2000 the previous 10 October, after which it became 15 October from 2001.
