Once a General Election is called or the by-election process has been started, set electoral timetables apply.
These are laid down in the Parliamentary Election Rules contained in the Representation of the People Act 1983.
The basic rule is that a general election must take place on the 17th day, excluding Sundays and holidays, following the date of the proclamation summoning the new Parliament.
In recent times this has been the same day as the dissolution of the old Parliament.
The timetable for by-elections is flexible - within certain limits - whereas the General Election timetable is more rigidly set.