I often say that this kind of movie does not deal well with reality. This premise is one of the more ridiculous in that way that I can remember. So let's try to get over it.
Now the premise is that two old high school flames have to work together on a Christmas (sort of) project. That is one of the more common ones. In other words, the story is just a platform to allow a romance to develop.
There is some decent dialogue including some funny lines. The acting is good. Based on the set-up, the lead protagonists should be about as antagonistic as they could be. While there is some residual friction over business matters and personal history, they get awfully friendly awfully fast and soon the friction turns to nostalgia. Danica McKellar and Neal Bledsoe do have very good chemistry.
There are some little mysteries that unveil later in the movie. Early on, both Sadie and Holden tell their side of their breakup in high school, but it's obvious that something is missing. About half way through we learn more about both backstories since high school.
I said let's try to get over it. Unfortunately, the movie keeps bringing the ridiculous premise up, like rubbing it in the viewer's face. It is central to the story all the way through. And the miracle is even more ridiculous. I really wish the premise hadn't been so stupid. I might have been able to see this as a slightly above average movie despite being entrenched in overused tropes and with no real surprises. But as is, the ridiculous story just takes away too much.