They call it St Totteringham's Day. The name sounds medieval, as though it dates back centuries. In fact it only seems to have been coined in 2002.
It is the Arsenal fans' term for the point in the footballing calendar when it becomes mathematically impossible for Tottenham to finish ahead of them.
Now there may need to be an addition to the footballing lexicon for its opposite: the day when it becomes certain Spurs will end the campaign as North London's top team.
For 21 consecutive years, they have trailed their neighbours in the final standings. Now an era could end tomorrow. Win and Arsenal cannot overhaul them.
Arsene Wenger seems set to lose two immaculate records. He has always finished ahead of Tottenham, and always put the Gunners in the top four. His preoccupation now should not be to salvage statistical perfection, but to illustrate that he can engineer a revival after the worst run of his reign.
His switch to 3-4-2-1 suggested that, at 67, Wenger can still come up with new ideas. The comeback victory against Manchester City in the FA Cup indicated that Arsenal can show the spirit they were long accused of lacking to win the sort of major match they have tended to lose.
Perhaps, as reported, the players are still indeed delivering for Wenger.
And instead the focus switches to Tottenham. While both sides won 1-0 on Wednesday following mid-match tactical switches from their managers, their FA Cup semi-final defeat to Chelsea prompted Harry Kane to say they need to triumph in big games.
Their only three league defeats have come to top-five sides. They have only nine points from a possible 24 against their top-six peers. They have won only seven of 49 derbies against Wenger's Arsenal.
On the other side of the equation, however, is Tottenham's outstanding record at White Hart Lane this season and Arsenal's wretched record on their travels in 2017.
Wenger has prospered with his new-look system but Spurs have played it better. They have been a team with more coherence and dynamism of late. They have more players near the peak of their game and more certainty about their future, while the issues of whether Wenger and Alexis Sanchez will stay at Arsenal remain on the agenda.
They have the division's best defence and, in the newly-crowned PFA Young Player of the Year Dele Alli and Kane, they also have two players who have outscored anyone in an Arsenal shirt in the Premier League in 2017.
Form renders them favourites. History suggests Arsenal have a chance.
But while derbies can obscure everything else, Tottenham's bigger preoccupation should be whether they can catch Chelsea.
Arsenal surely will not finish above their neighbours, but they could prolong Spurs' 56-year wait for a league title.
TOTTENHAM V ARSENAL
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