I would pretty much bet a million bucks on the fact that the christmas party was on the 14th. My hubby has been in the Army for 15 yrs and we have been together 10 years and we have never attended a christmas party on the weekend.
In my experience, They usually let everyone off at noon (half day) and then everyone (military) is usually expected (mandatory) they show up at the Christmas Party, don't really have to stay long, just pretty much show your face. Alot of the times for the lower enlisted from my experience, it isn't such a big deal if they don't show up, compared to the senior nco's. .
Maybe that is why Xtina went to the party and he didn't. Possible that she showed up and said he is not feeling well, he had to take care of something, whatever she might have said. If she had been in the marines, she was probably comfortable at a party without him there. Maybe she went so people wouldnt wonder where he was or maybe if they seen her they would assume he was there somewhere too.
Every year for hubby it is usually a unit xmas party and there are atleast 100 people there. I know most of the times everything is so chaotic with all the kids running around that if I see a spouse, then I just assume their hubby or wife is running around socializing.
She could have told anyone who asked...."he is around here somewhere", maybe this is what gave them more time without people suspecting anything. So maybe she was covering for him, while getting their daughter out of the house as well, so he could do whatever they had planned.