Mark down Dec. 4, 2017, as a seminal day in New York Giants history.

In a sweeping strike, Giants ownership fired general manager Jerry Reese and head coach Ben McAdoo, a Monday massacre that will forever change the history of a franchise that has lost its way.

Co-owners John Mara and Steve Tisch came to this conclusion Monday morning, the day after their Giants lost 24-17 to the Raiders, dropping a team that was expected to qualify for the playoffs and challenge for a Super Bowl berth to 2-10.

“We agreed that wholesale changes needed to be made to this organization to get us back to the team we expect to be,’’ Mara said. “We also agreed it was pointless to wait any longer to make these changes.’’

Defensive coordinator Steve Spagnuolo will serve as the interim coach for the final four games and will determine if Eli Manning returns to start at quarterback after his streak of 210 consecutive starts ended in Oakland. It is widely anticipated Manning will start Sunday against the Cowboys, replacing Geno Smith. Kevin Abrams, the assistant general manager, assumes Reese’s duties until the end of the season.

Having decided weeks earlier that a purge was inevitable after the season, Mara took a look at the schedule, fully realizing three of the last four games are at MetLife Stadium, and that the fans, seething about this season and stunned by the way McAdoo started Smith instead of Manning, would protest and create an intolerable situation for McAdoo and the franchise.

“I’m conscious, having lived through it before, of what the reaction was going to be,’’ Mara said, adding making these moves now, “gives us somewhat of a tactical advantage allowing us to start looking at general managers right now, rather than waiting until the end of the season.’’

Mara has hired former Giants general manager Ernie Accorsi as a consultant to help with the general manager search.

Mara insisted the way McAdoo handled the Manning Affair — offering him a chance to start before benching him after halftime, a plan Manning rejected — was not the spark that ignited this firing.

“Really had no effect whatsoever,’’ Mara said. “Two-and-10 is 2-10 and obviously the public reaction to that was not pleasant, but that really didn’t have any effect on our decision.’’

Mara met Reese around 10:30 a.m., calling it “as difficult a meeting as I’ve ever had.’’ Reese was part of the organization since 1994 and during his 11 years as the general manager, the Giants won two Super Bowls. This will be the fifth time in six seasons, though, the Giants will fail to make the playoffs.

“I don’t think there was any one final straw,’’ Mara said. “I just think that where we are as a franchise right now, you know, we’re 2-10. We’ve kind of been spiraling out of control. I just felt like we needed a complete overhaul.’’

Mara said he told Reese, “I have no doubt that he will get another shot with another franchise and that at some point in time, I’m going to be answering questions from people about why I got rid of him in the first place.’’

Next, Mara met with McAdoo, who could not have been surprised by the news presented to him.

“I feel that he will be a successful head coach at some point in the future,’’ Mara said. “I think he’s going to learn from his experience here and he’ll go on to be a successful head coach.’’

The Giants had not fired a general manager since 1978.

McAdoo’s tenure lasted 28 games, making this the shortest stay for a Giants head coach in 89 years. He has two years remaining on his contract. One year after he was a Coach of the Year candidate after going 11-5 in his debut season and leading the Giants to their first playoff appearance in five years, McAdoo exits with a record of 13-15, plus an 0-1 record in the postseason.

The last time the Giants fired a coach during the season was 1976, when they parted ways with Bill Arnsparger after an 0-7 start.

A 31-21 loss to the previously winless 49ers on Nov. 12 prompted ownership to issue a statement that all evaluations of the coaching staff would be made after the season.

“We changed our minds,’’ Mara said. “Given all the events that occurred, where we are as a franchise right now. To be honest with you, it became more and more apparent that we were going to have to do something at the end of the season, so we talked after the game and again this morning about, why prolong it any longer? Why not just get it done now?’’