Fixed term Parliaments - they didn't think it through properly.

Before everybody drifts off topic in the comments, as a general rule, I'm all in favour of fixed term governments, most other civilised countries seem to have them. That's not the point here.
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If you ask me, the reason that the current government decided to have five-year fixed term parliaments is because the elections to the European Parliament are held every five years and have been since 1979. In 1979, Jim Callaghan had to call a General Election because their five years were up, but since then no UK Prime Minister has called a General Election in the same year as a European election*.

They have switched between calling an election after four years (if things were going well for them) or hanging on for five years (if things were looking bad for them), but carefully avoided having a General Election in the year that a European election was due to happen (starting, ironically, in 1983 with Thatcher, thus avoiding a clash in 1984).

UK political parties spend a lot of money and effort on their election campaigns, so having a European election and a General Election in the same year would completely exhaust them, so over time, an unspoken agreement has been reached whereby they use the European elections as a dry run for the General Election, in other words, they hold the General Election the year after the European election (which happened in 2004/2005; 2009/2010 and 2014/2015). Having been through this mill, it's a good way of doing things.

This all came to a head in 2010. There was a European election in 2009 (so the next one is due 2014) and the Lib-Cons won the General Election in 2010 (so the next General Election could have been in 2014 or 2015). As the two coalition parties and the Labour opposition are pro-EU and do not countenance leaving it in their wildest dreams, they declared an armistice and agreed in advance that the next General Election definitely would definitely not be held in 2014; it would be postponed until the year after the next European elections i.e. in 2015.

So the next European elections will be in 2019 and the next General Election will be in 2020, everybody knows what they are doing.
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So far so good.

What they forgot was the four-year cycle of the elections for the Scottish and Welsh Assemblies**, which were held the first time in 1999 in Scotland and in Wales. So far there hasn't been a clash between those and the UK General Election.

But of course, sooner or later the pre-existing four-year cycle and the new five-year cycle have to coincide, which happens to be this year 2015. And the parties don't want that either. So the Scots and Welsh duly agreed to defer their elections until 2016, a year after the General Election.
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Again, so far so good - but here's the fun part...

If the Scottish and Welsh Assemblies have elections in 2016 and then want to have them four years later in 2020, that will clash with the General Election due to be held in 2020, so they'll have to defer until 2021 and so on until the end of time.

They might as well have just decided to extend the lifetime of the Scottish and Welsh Assemblies elected in 2011 by a year and gone straight over to five-year elections.

The same applies to the Greater London Assembly, which has had a four year cycle so far (2000, 2004, 2008, 2012) none of which clashed with the General Election, but that is going to happen in 2020, so the 2020 GLA election will no doubt be postponed until 2021.

* This is a complete misnomer, but I will use it for convenience.

** The Scottish Assembly has started calling itself the Scottish Parliament, I don't know how that happened.