UK elections for local government and national government are held on Thursdays. Politicians whinge about the public not turning out for elections; whilst there are many reasons why this is (chiefly the politicians themselves being perceived as power-crazed crooks), one possible reason is the inconvenience of Thursday elections.
There is no statutory requirement for elections to be held on a Thursday, although they must be held on a working day. The exception are local elections which must always be held on the first Thursday in May unless the Home Secretary decides otherwise. See the relevant page on the Parliment web site.
The official explanation still does not explain why Thursdays are so important, although unofficial explanations include :-
Obviously except for the first reason, these are no longer valid reasons for holding elections on Thursdays. Now it is just a long held custom.
As to why it is necessary to hold elections on a working day, the cynic would believe (quite possibly correctly) that this is to make it less convenient for working people to vote. Thus allowing the ruling classes continued excess influence.
Or to be more precise why any normal working day is bad.
The opening hours of a polling booth are from 7am to 10pm which would seem like plenty of time to give anybody an opportunity to vote ... at first. It isn't that these hours make it impossible for large numbers of people to vote, but it isn't always easy either. People who are deeply interested in politics will easily manage to find the time to vote. If however you are one of the majority that at best votes for the politican that you dislike the least then you are less likely to find the time to vote.
If voting were to take place on a day when most people are not at work ... perhaps Sunday (although many people do work on Sundays), then there is a chance that those who don't usually vote will. After all, with a whole day free, there's plenty that could find 1/2 an hour to vote.