The trading window is the window of time each weekday when M1 makes all orders for user accounts. M1's trading window benefits users because it helps keep M1's management fees low since M1 is only trading one time per day. M1 is a long term investing vehicle, not a trading platform, so timing of trades are less important.
M1's trading window occurs at 9am CT everyday the NYSE market is open. All orders placed before 9am CT on days that the NYSE is open are executed the same day during M1's trading window.