Toby Manhire undertakes a very possibly foolish attempt to whittle down the dates to the likeliest polling day, based on All Blacks, Trump, school holidays and the vibe.

I feared I might have missed something when on Sunday a newspaper preview of the year noted “the general election in November”. That same day, in another publication’s preview of a big year for elections around the world, a sentence began, “In New Zealand, the election on Nov 21 …” It went on to mention “Jacinta Ardern”, so might not be the greatest of authorities, but still I wondered: had the prime minister quietly announced this year’s polling date?

Further research suggested she hadn’t, but turned up a likely culprit: popular search engine Google. For if you plug in “New Zealand election 2020”, it volunteers the following:

Sorry, popular search engine Google, but you’re wrong. It’s possible of course you can see into the future, but more likely you just crawled the text in the Wikipedia entry on the NZ election 2020 – “The last possible date for the next general election to be held is 21 November 2020” – and trimmed it down to an easy burst of (wrong) preview info.

So when then can we expect election day, which comes this year with a pair of referendums, on cannabis and assisted dying, to take place?

It is true is that November 21 is the latest date an election can be held, based on our three-year cycle. Jacinda Ardern has indicated she’ll follow in the recent tradition of John Key and Bill English in announcing an election date in the early part of election year, so it’s unlikely to be long before we know the chosen Saturday, but in the meantime, let’s speculate.

Of course, it’s in the power of the prime minister to call an early, or “snap” election. Some were urging Ardern to do so last year, but she dismissed the idea. Going early in what is already a short term hardly exudes confidence. So it’s a pretty solid bet that Ardern will nominate a date at some point in the three months leading up to November 21.

In the eight elections since MMP was introduced, there have been three September elections and three November elections. The first MMP election, in 1996, was held in October. And in 2002 it was late July, with Helen Clark going to the polls early citing the collapse of the Alliance.

A couple of other things to bear in mind. Advance voting is an increasingly big chunk of ballots cast. At the 2017 election, advance voting surged to more than 1.2 million votes, meaning that almost half of all votes were cast in the fortnight before polling day; that changes the nature and pacing of the campaign. And while the election night tally can be decisive, the special votes can be enough to change the nature of any coalition negotiations. Winston Peters was right to say he needed to wait for that number to come in last time; specials made a Labour / NZ First plus Greens government significantly more plausible. All of which means that negotiations may not begin in earnest until a fortnight after polling day.

But enough of that. Let the speculation begin.

Late August to November 21 gives us 13 Saturdays:

August 29, September 5, September 12, September 19, September 26, October 3, October 10, October 17, October 24, October 31, November 7, November 14, and November 21.

We can begin by binning the first two. Partly because – especially given advance voting – they’d push the campaign solidly into the miserable winter month of August. And partly because of a traditional portent in election date prediction: rugby. There are home All Black tests on the evenings of August 29 and September 5.

That leaves:

September 12, September 19, September 26, October 3, October 10, October 17, October 24, October 31, November 7, November 14, and November 21.

The other reliable guide to when an election will – or, rather, won’t – be held is school holidays. With thousands of families away from home, voting is made less convenient. The 2020 school holidays span the Saturday of 26 September and the first two in October. Out they go. And let’s while we’re at it be rid of the Labour weekend Saturday, 24 October.

Seven remain:

September 12, September 19, October 17, October 31, November 7, November 14, and November 21.

How about what’s happening internationally? There’s quite a lot on. The expo in Dubai, in which New Zealand is participating, is kicking off late October. The Apec Summit in Malaysia takes place in November – likely to be the second weekend – and ideally you’d want a freshly confirmed prime minister to attend that, which would mean getting negotiations wrapped up tidily. It’s not unmissable, but it takes on greater importance given New Zealand is hosting the following year.

There’s also the small matter of a presidential election in the United States of America on November 3. An election over there doesn’t of course proscribe an election over here – the last time they fell in the same year, 2008, the New Zealand election took place four days after the Americans’. But even the Obama victory didn’t suck the oxygen up the way Donald Trump does.

Put all that together, and add the risk of negotiations riding up into Christmas week, and we’re going to brazenly cross out everything from October 31 onward.

And then there were three:

September 12, September 19, October 17.

September 12 teeters the campaign just that bit too far towards the short, wet days of August.

October 17 is tempting, and a pretty decent shout. But it does mean two of the crucial final three weeks are during school holidays, and there are those early November dates on the international calendar.

So my guess is that the election will, in keeping with the last two, fall on the the second to last Saturday of September, which would mean a New Zealand general election 2020 on:

September 19.

Which, by happy coincidence, would be the 127th anniversary of New Zealand becoming the first nation in the world to grant women the right to vote.