Greetings Karen:
I would wait until late winter/early spring to plant these bareroot seedlings. Theoretically, you can plant trees anytime as long as the soil isn't frozen (planting in frozen soil is difficult to do). Of course, you would likely avoid planting trees during periods of severe drought and high temperatures.
If you can wait, then hold-off until the soils warm back up in order to prevent heaving (i.e., lifting of the seedlings after planting). If you can't wait, maybe because you have the seedlings in your possession and you do not have means to cold-store them at 34-38 deg F @ high relative humidity, then I suggest putting the seedlings in the ground 'temporarily' to prevent dessication and root exposure.
