When it happens, it sends shock-waves through the building. The reverberations are felt in the locker room and the training room and the cold tubs and the practice fields, in the team cafeteria and players’ lounge and meeting rooms. Everywhere.

Coaching changes are woven into this time of year; the Holiday Season is also the Horrible Season when it comes to uprooting families and altering the way of life inside a football operation. The last game is played and offices get cleaned out, the one-time inhabitant told he cannot return to a place he spent hour upon hour, day after day, trying to get right what ultimately went wrong.

These could be the final days for Pat Shurmur as the head coach of the Giants, and for all the hue and cry to get him out, a pause is needed to sing the familiar be-careful-what-you-wish-for refrain. Continuity and stability are the hallmarks of any successful franchise, and the upheaval that enters the room when an entire coaching staff heads for the exit is real, disconcerting and counter-intuitive to long-term winning.

“Everything is different,” receiver Sterling Shepard told The Post. “Rules change throughout the building. What you can wear, what you can’t wear. All that changes. It’s tough. And you hate to see it. We’re all grown men, we all have family to take care of and we all touch each other’s job in some way or another. You feel like you could have done better for that person, if that were to happen. Speaking for myself, I feel that sometimes, where I felt like I could have done better for that person.”

It is never one man, of course, and that has to be remembered and reinforced whenever analyzing whether Shurmur stays (unlikely) or goes. If ownership decides a new voice is needed after Shurmur went 5-11 in 2018 and takes a 4-11 record into Sunday’s finale against the Eagles, it will highlight a particularly dysfunctional period in team history.

Ben McAdoo did not make it through two full years of his four-year contract and now comes off the financial books. Shurmur signed a five-year deal and that is part of what must be weighed when his future is discussed. Paying someone else not to coach for three more years, after paying McAdoo the past two years, is an embarrassing potential reality for owners John Mara and Steve Tisch, not only for the nearly $20 million it could cost them.

Shurmur has 21 assistants and it is believed ownership is not high on some of the members of the staff he assembled. Coaching matters. There is a sense this group does not give the Giants an edge as far as player development and game-day execution. Still, there is talent working in the building and any new man in charge will bring in his own people.

“It’s the NFL, you have coaches change all the time,” Shepard said. “It doesn’t necessarily have to be a head coach you change. It could be your position coach. You build a relationship with somebody. I built a relationship with [former receivers] coach [Adam] Henry; I still talk to him every other day, and the same with coach Tyke [Tolbert]. No one likes that part of this whole deal.”

Shepard is only 26 years old and yet he is an elder-statesman as far as his Giants longevity. Long-snapper Zak DeOssie and franchise icon Eli Manning will not be back and Janoris Jenkins was already waived and signed by the Saints. Shepard arrived in 2016 and will be the longest-tenured Giants player in 2020.

Already, Shepard has seen McAdoo hired and fired and may outlast Shurmur, which would put Shepard in his third offensive system in four years. He says he does not really sense any brimming anxiety with Shurmur and his staff.

“They come to work every day trying to prepare us to win the game,” Shepard said. “I can’t say I’ve seen anything other than that.”

If a change is made, Shepard has experience with what comes next.

“One, you have to come back earlier than you would have to if you didn’t get a new coach,” he said. “Two, you have to worry about picking up a whole new system, building new relationships. It’s a process but that’s kinda the way this thing goes.”

It is the way the thing goes when a franchise is failing the way the Giants have failed for too long now.

For more on the Giants, listen to the latest episode of the “Blue Rush” podcast: