ON Tuesday, after I'd given a talk to the Darlington Retired Teachers' Association, one of the few people left awake asked a devastatingly simple question: "On what grounds do you media types keep telling us that the General Election is on May 5?"

Desperate to sound knowledgeable, I flim-flammed a fabrication of an answer. But it was pretty flimsy, and further explanation is required.

Particularly as, according to the Septennial Act of 1715, Parliament can last for five years from its first meeting. This means that Tony Blair could wait until June 12, 2006, before calling an election at the absolute latest on July 6, 2006.

Of course, he won't.

In recent times, only John Major has toughed it out for the full five years, but that was because he expected to lose in both 1992 and 1997.

Wherever possible, Prime Ministers opt to go in their fourth year as it leaves them a little room should something unexpected blow up. And, conventional wisdom has it that most unexpected things blow up in the summer - the fuel protest, for example - and so it's better to go in the spring.

But you have to go early enough so that people aren't away on holiday, and you have to go late enough so that a sudden fall of snow doesn't stop them getting out to vote.

You have to allow time for your Chancellor to have his Budget debated and accepted. Direct taxes - income tax and corporation tax - are 'temporary' and have to be renewed every year otherwise the Government only has money coming in from indirect taxes, like fuel duty, which are 'permanent'.

So, for weather and Budgetry reasons, the poll will be after March, and for holiday reasons, it will be before July. And because Mr Blair wouldn't want to be campaigning in the run-up to the Royal Wedding - the country will, of course, be in such a heightened state of excitement in the weeks before Charles and Camilla get hitched that it won't be able to concentrate on mundane matters like politics - his poll will have to wait until after April 8.

A further complication is that since 1935 every General Election has been held on a Thursday (the last non-Thursday election was on Tuesday, October 27, 1931).

Now the dates are narrowing down. Especially as on Thursday, May 5, many areas of the country are holding local elections. Mr Blair won't want to be accused of wasting public money by having two election days in quick succession when he could have had one.

The answer becomes even more obvious when one considers that last year, Cherie Blair told the Cheltenham Literature Festival that she had cleared her diary for May 5, and that Konstantin Kosachyov, chairman of the Moscow State Duma foreign affairs committee, said Mr Blair had declined an invitation to Moscow on May 9 because of a General Election a couple of days earlier.

The real clincher is that on November 24, the Sun newspaper announced that May 5 was the date. Downing Street didn't dare demur.

Because an election must be held on the 18th working day after it was called, we can say that Mr Blair will formally announce the date of the election on April 4. Parliament will be dissolved on April 7, and the writ summoning the new Parliament, and so triggering the election campaign, will be signed and sealed on April 11.

Definitely. Unless, in a fit of pique, late last night Mr Blair called a snap election. In which case, please disregard all of the above as ill-informed flim-flam.