May 13, 2016, to be precise after Albion had suffered four in-game injuries, been reduced to ten men and lost 2-0 to a fit, rested and hungry Sheffield Wednesday side in the play-off semi-final first leg.
Albion were mentally and physically shattered going into that game after their heartbreaking near miss for automatic promotion.
What was overlooked by joyful Wednesday fans as they celebrated after the first leg was they had actually missed a trick.
They should have buried Albion that night. Killed the tie. Won by four or five.
At full-time, Rosenior went to the away end showing the fans it was only two and telling them to keep their chins up.
Although a magnificent effort came up short in the return, partly because of a Wednesday goal which should have been disallowed for a foul, those defiant gestures from Rosenior are still remembered by Albion fans.
What I recall just as vividly is what happened six days previously at the Riverside, away from the fans' view.
Albion needed a win, Middlesbrough needed a draw in the race to go up with Burnley.
Boro led, then Dale Stephens equalised but was sent off soon after by Mike Dean for what looked a yellow card challenge.
No Albion player felt like talking after that game. There was actually nowhere to do interviews because all areas behind the scenes had been flooded with fans and players’ families.
But Rosenior stopped for us near the team bus and addressed pre-game comments by Boro boss Aitor Karanka, who had questioned Dean’s appointment for such a big game.
Rosenior spoke about respect and the way things should be done.
He stood Albion’s ground, fought Chris Hughton’s corner.
Amid the Teesside celebrations going on all around us, he said: “It starts before the game.
“It starts with managers who, for me, have no place talking about who should and shouldn’t be refereeing a game.
“That’s one of the reasons why I respect this manager (Hughton) – because he never degrades himself to that level.
“The English game is all about respect. Respect for each other.
“I’ve got massive respect for Middlesbrough but I wouldn’t dream of talking about who should and shouldn’t be reffing a game of football.
“It’s a difficult job as it is.
“When you try and influence decisions like that, I think karma will get you back in the end.
“Good luck to Middlesbrough but I think a few people at this club could show a little bit more respect to the English game.”
Almost a decade on, we all know what has happened since then to the clubs who were involved in that end-of-season drama.
We can see the current status of Albion, who missed out, champions Burnley, promoted Middlesbrough, play-off winners Hull and Wembley finalists Sheffield Wednesday.
Rosenior arrived at the Amex at what might be termed the start of promotion campaign in the summer of 2015.
Yes, the word “campaign” is usually used by football writers as an alternative for “season” but it has a wider meaning than that.
A campaign is a mission or project over a certain period of time aimed at achieving a clear goal. Like an election campaign.
Using that logic, Albion’s promotion campaign started in July 2015 and ended late on Easter Monday, 2017.
Rosenior did not play the part he would have wanted on the pitch, partly due to injury but also because he pushed Bruno to new levels.
The two were similar figures of experience on the pitch and, along with their wives, became good friends off it.
By this weekend, both will know what it is like to have been on the bench as head coach of Chelsea.
The BlueCo bosses have appointed Rosenior as new boss of their Stamford Bridge branch on a deal until 2032.
The 41-year-old revealed earlier on Tuesday morning during a farewell press conference at Ligue 1 club Strasbourg that he had “agreed verbally” to replace Enzo Maresca at Stamford Bridge.
Rosenior will be equipped with a lot of lessons learnt in the past decade as coach at Hull, Derby, Strasbourg and, before that, helping with Albion under-23s.
If leadership and character in the midst of heartbreak and adversity are needed, we have seen that as well.