There's not one answer to when is it the best time. Many parents, for instance, don't want to sell and move in the middle of a school year. However, some people find that because there are less listings in the fall and winter, that there are less homes competing with their home, and supply and demand says that if there isn't enough inventory (and there hasn't been enough inventory on Long Island for the last 12 months now), prices will rise.
One thing that certainly hasn't changed this fall is that first time homebuyers are still out in force. Remember, not every buyer is tied to the school schedule. Interest rates are still very low, and the private mortgage insurance rates were halved in February 2015. That said, rates last week climbed to a 4 month high, and with the election wrapping up, it will take much of the uncertainty out of the rate markets, which we see as leading to a steady rise in rates over the next 6 months. If you wait till spring to sell, you may find that your potential buyers may be able to afford less home, which may start to retard the growth of home prices.
Those two factors of locking in while rates are still low and maximizing how much house you can get for your money mean that the buyers who are out right now are the most serious ones, and there are enough of them to get you the best price possible.