Get used to it. From now until the announcement is made — which could be as much as six months off — there will be incessant speculation on the date of this year’s federal election. Pretty much every week there will be some turkey expecting an early election to be called.

Most democracies don’t have this problem; they have fixed or semi-fixed terms, where the election date is set well in advance. But part of our legacy from the Westminster system is the prime minister’s ability to choose an election date to suit his own political convenience.

That’s not to say that John Howard has a completely free hand in the matter. (Spare a thought for Ireland, where the prime minister even gets to choose what day of the week an election is held.) There are a number of constraints on the election date, which are worth a brief explanation. (The parliamentary library has a more extensive account of how it works.)

First, in practice no election can be held before August. This results from the conjunction of three points: no government wants separate half-Senate elections, so the House of Representatives and half-Senate elections will be held together; the half-Senate election can only be held after 30 June; and there is legal advice (not conclusive, but not worth risking) that that means the writs must be issued after 30 June, which means the election itself would be a minimum 33 days later.

So the earliest possible election date is Saturday 4 August. If Labor suffers some unimaginable catastrophe in the next two months, an August election will be a possibility. Failing that, it’s extremely unlikely; being made to turn out to the polls in winter is not likely to fill the electorate with enthusiasm, which probably explains why only one August election has ever been held, back in 1943.

September has a different problem: the APEC summit, to be held in Sydney on 7-9 September. That certainly rules out Saturday 8 September, and realistically the previous Saturday as well. It also means that an election couldn’t be held in the following four weeks without the campaign extending over APEC, which would be a serious breach of convention.

Granted that this is a government not shy about breaching conventions, a September election still seems out of the question. October 13 would be possible, but only by running on the minimum timetable and calling the election the day after APEC finishes; that would look a bit desperate. The following week, 20 October, seems much more likely: pencil it in as one of the two best chances.

From then through November, any weekend is possible. But if Howard doesn’t choose the October option, it will presumably be because he thinks he needs more time to try to wind back Labor’s lead, and if that’s the case, he might as well take as long as he can.

The latest theoretical date is 19 January 2008, but a campaign over Christmas is unthinkable. No election since the 1930s has been held later than 13 December (the post-Dismissal election of 1975), so the latest practical possibility is Saturday 8 December. Mark that down as the other most likely date.