For a sortie to fly it needs several things. First of all, the planes have to match the mission. If you want to fly night missions, the planes have to be set as well. Planes can be set for DN (Day/Night), DO (Day Only), NO (Night Only), RS (Resting), TR (Training).
When you select your Air Directive, you can set the mission to fly either DAY or NIGHT.
“My logisticians are a humorless lot … they know if my campaign fails, they are the first ones I will slay.” – Alexander the Great