The one thing we know for sure is that Prime Minister David Cameron has said it will happen by the end of 2017. The most likely times of the year for referendums are generally May or September, and some people - including, it is said, the prime minister himself - think it should be held as soon as possible. There had been suggestions that it could be held in May 2016, to coincide with elections in Scotland, Wales, Northern Ireland and London, rather than waiting for 2017 - but the government has ruled that out and June or July 2016 are now seen as most likely.