Hey. It gets better.
Tonight, my wife is on a date with her (our) other partner, I'm at home with the kids, kind of bored, kind of distracting myself, kind of keeping on while really wishing I was there with them. (Okay, my situation is different from yours in that we're a pretty tight triad, but the feeling of being alone at night - tonight - is pretty similar.)
There are a bunch of resources on the site, meaning a bunch of stories of folks in your shoes. Something to think about: if you're really happy that this is happening, focus on that. If you're anxious about what she's doing, name it and work out the worst case scenario. It won't turn out that way. Trust me: the worst case scenarios are absurd; we humans are built to think of absurd worst case scenarios. If you're feeling jealous, ask yourself what is going on that you wish you were part of (date night removed from the cares of your marriage? arrange one with your wife!).
The point is, there are lots of solutions to the anxieties that you're feeling. Nothing connects you to your raw self like moments like this. It's not easy, but it's worth the effort.
And when she comes home, greet her in peace and calmness. Let the love you have speak out. Ask her, without fear, if she's happy - if you need details get them, if you don't need them, kiss her as hard as you can, and find to your strength with each other. She didn't leave you tonight, she just happens not to be there. It's not that big a deal. It really does get better.