The Family Man

Jack Campbell is a single Wall Street executive living in New York City. He is in the midst of putting together a multi-billion dollar merger and has ordered an emergency meeting on Christmas Day. In his office, on Christmas Eve, he hears that his former girlfriend, Kate, called him after many years. He walks into a convenience store where a lottery contestant, Cash, has a winning ticket worth $238. The store clerk does not believe Cash, so Cash pulls out a gun and Jack offers to buy the ticket. Jack offers to help Cash. In return, Cash questions Jack, asking him if he is missing anything in his life. Jack says he has everything he needs. Cash tells Jack that actions have consequence and that Jack has brought whatever is coming on himself. Jack returns to his penthouse and sleeps.
On Christmas Day, Jack wakes up in a suburban New Jersey bedroom with Kate and two children. He hurries back to his office and condo in New York, but his closest friends do not recognize him. Jack runs out to the street and encounters Cash driving Jack's Ferrari. Cash explains that Jack is experiencing a glimpse of an alternate universe in order to learn a lesson. He advises Jack to take the time to learn whatever it is that he needs to learn.
Jack is living the life he could have had, had he stayed in the United States with his girlfriend. He has a modest family life, where he is a car tire salesman and Kate is a non-profit lawyer. Jack's young daughter realizes his secret, thinks he is an alien and decides to assist him in surviving his new life. Jack struggles to fit into the role of a family man, such as missing opening Christmas presents, flirting with a married woman and forgetting his anniversary. He begins to succeed in his life, bonding with his children, falling in love with his wife and working hard at his job.
At a chance meeting, he is offered a contract to work at the same investment firm from his real life, having impressed the Chairman with his business savvy when he came in for a tire change. His old mentor offers him a job, while a formerly sycophantic employee is in Jack's old position, with an assertiveness he did not possess as a subordinate. While he is wowed by the potential salary and other complimentary extreme luxuries, Kate argues that they are very happy and they should be thankful for their life.
Just as Jack is finally realizing the true value of his new life, he sees Cash again as a store clerk and demands to stay in this life, but Cash informs him that there is nothing he can do. His epiphany jolts him back to his wealthy former life on Christmas Day. Jack forgoes closing the pharmaceutical acquisition deal to intercept Kate. He finds her moving out of a luxury townhouse. Like Jack, she also focused on her career and became a very wealthy corporate lawyer. She only called him to give back some of his old possessions. Before she moves to Paris, he runs after her at the airport and describes the family they had in the alternate universe in an effort to win back her love. She agrees to have a cup of coffee at the airport, suggesting that they will have a future.

Question: Which couple's event did Jack forget?
Answer: his anniversary