Corsaire, I was a little confused myself and Olham answered it in an earlier post, which there are a lot and easy to miss.

You can fly however many missions your squad flies on that day. Or you can fly 1 if that is all you want to fly. Once the time advances to the next day, you wait till then to fly again. I flew 2 missions on the 9th, was wounded on the second mission and will not fly again until the 14th when I get out of the hospital. If your flights for say the 11th of June are cancelled for bad weather, you don't fly on the 11th, try again on the 12th. Hope this explains it.
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