May 13 is a date imprinted on the mind of every Mancunian. For those on the blue half of the city's great footballing divide it is the day when in 2012 they went from an incredible low to an awesome high. Just as their team was about to let the Premier League title slip from their grasp they won the thing in the most dramatic climax ever to a top flight season in England.
This year the Old Trafford club did just that, with their success marked with an open-top bus parade into the city centre yesterday evening – exactly a year after the Blues had beaten the Reds to the win the race.
The date that is so memorable to City fans that they christened it 'Aguero Day' will now be tinged forever. On the first anniversary of that thrilling triumph the manager who led City to the only silverware that the club has won in the last 37 years was sacked.
The joy of those bedecked in red and white as their heroes displayed their latest piece of silverware as they travelled from Old Trafford to Manchester's Albert Square was contrasted by their rivals' fans frustration.
How could they go from the greatest day in the history of the club to sacking the manager in the space of 365 days?
The days of 'Typical City' when the club shot itself in the foot over and over again were supposed to be consigned to history when captain Vincent Kompany raised the Premier League trophy above his head this time last year, but in the manner of Mancini's dismissal history was repeated.
The circumstances in which he was installed in December 2009 caused the club to admit that mistakes had been made. Even though they have billions behind them the Abu Dhabi owners obviously haven't learned the lessons.
Stories that he was about to be axed hit newspapers on the morning of the game.
The sacking of a manager is never greeted with universal acclaim, but there is a right way and a wrong way to do it and City haven't got it right under the current ownership yet. David Moyes will begin work as Sir Alex Ferguson's successor as Manchester United manger next Monday, 47 days before the start date of his contract.
Ferguson's shuffling off was supposed to be United's moment of weakness and the chance to puncture the aura of red invincibility in their town, but what did City do when their tormentor-in-chief finally disappeared towards his "bath chair on Torquay beach", as he once called it? They blew it.
Before Mancini was finally sacked last night, we saw a symbol of City's sense of drift at Loftus Road, where the squad's training session ahead of tonight's match with Reading was overseen by David Platt and Brian Kidd, with Mancini in civvies and apparently not mightily interested.
The Italian was escorted out of the ground before leaving ahead of the team coach.
There was a sense all day, only ended by the axe eventually falling when the club announced the news in a statement on their website shortly before 10.30pm, that asking Mancini to take the side at the Madejski Stadium would be humiliating, with the writing on the wall.
Though there are plenty of reasons to go their separate ways with Mancini, there is a justifiable sense of bewilderment among some fans about appointing Pellegrini, a Chilean who, at 59, will be the Premier League's second-oldest manager, after the 63-year-old Arsène Wenger.
Granted, there is something rather irrational about the cult of the young manager. Ferguson was always very sniffy about it, arguing the merits of experience, and, in Moyes, United have someone who has experienced most of football's hurdles several times over.
But Pellegrini is yet to win a trophy in Europe, even after buying Cristiano Ronaldo, Kaka, Karim Benzema and Xabi Alonso at Real Madrid in 2009. Expect Pellegrini's English to be stronger than Mancini's was when he arrived, but at least the Italian could talk about titles and cups, rather than storming the latter stages of the Champions League before eventual elimination, which is Pellegrini's narrative with Villarreal and Malaga.
To use the Bluemoon forum as a barometer, fans seem to wish for one of those names you feel could help build a dynasty: Andre Villas-Boas, Frank de Boer or Michael Laudrup were abundant yesterday. The 42-year-old De Boer is the manager who was so impressive when he held court in the depths of Ajax's stadium before City played and lost there last October, to a modestly assembled team.
That City should not have selected a British manager almost seems to go without saying, though the absence of obvious names meant that Liverpool's Brendan Rodgers was fourth favourite last night, when Pellegrini went 1/10.
Set against these doubts is the impact Pellegrini had at Villarreal, which sounds extraordinarily like Roberto Martinez at Wigan. It was Pellegrini to whom Fernando Roig Alfonso, scion of the bathroom ceramics dynasty which owns Villarreal, turned when he wanted to imbue the side with a certain philosophy running through the club.
"It has always been the same idea. We like to keep the possession of the ball. We always had those kind of players," Alfonso said in an interview two years ago which revealed the enormous esteem in which he holds Pellegrini.
"If you go back over the past five or six years, it's always been Barcelona and us like that. Barcelona because they have won everything and because their potential is much bigger than us they happen to be the ones who are known for that style of game."
When Pellegrini left for the Bernabeu, Roig hired an outsider, Ernesto Valverde, who tried to change the system. It was a disaster. So he went back to the bootroom for a coach.
All of which explains why the educated purist Pellegrini is attractive to City's Spanish executives, but time and tide wait for no man in the Premier League. While United continue, City are starting all over again.