Perhaps this will be implemented in the future when the capability is there. AE currently doesn't have the ability to calculate seasonal demand, so instead, I believe it shows it as being year-round which removes the whole point of seasonal flights. If seasonality is implemented in the future, then I could see a case for it.
Choosing days also requires a feature that either simply doesn't exist, or would require a change to implement. In the case of AE, this is scheduling, which currently is basically hours/week. Demand doesn't really care what day they're flying there and back (it's basically consistent every day), so all that's really necessary is a basic flights/week thing. Adding specific dates in which a flight would be would require a major change — instead of just figuring out how many hours a flight takes up and using basic subtraction, you'd also need to figure out what days a flight is flying, schedule each individual aircraft day by day or hour by hour ensuring that flights can operate non-consistent everyday schedules, and do the same thing for fleets of hundreds of aircraft, dramatically increasing calculations. Some sims like Airlinesim have managed to do this, but considering they're a paid program which operates in real-time, they can allow for this. I have no idea how we'd implement this into AE.