[from the Library website] Since the general election in 2010, when Parliament was dissolved by prerogative powers and the election timetable lasted 17 days, two pieces of legislation have made changes to the triggering of, and timetable for, a general Parliamentary election.
The Fixed-term Parliaments Act 2011 fixes the date of the next general election as 7 May 2015, and provides for five year intervals between elections. The Act ends the prerogative power of dissolution. The Electoral Registration and Administration Act 2013 made provision for lengthening the UK Parliamentary election timetable, both for general and by-elections. These provisions have been brought into force and Parliament will have to be dissolved 25 rather than 17 days before the general election on 7 May 2015. The by-election timetable will now also vary between 21 and 27 days from the issuing of the writ. For further details see Library Standard Note 6609, By-election timetables.
A brief outline of the timetable for the 2015 General Election is provided in this Note. Parliament will be dissolved on Monday 30 March 2015, Day 0 of the timetable, for polling day to be on Day 25, Thursday 7 May 2015. The Electoral Commission has recently published full details of the timetable for 7 May 2015 on its website.