Answer: According to Michael Dirr's Manual of Woody Landscape Plants, this tree (Albizia julibrissin) is injured at temperatures below minus 5 degrees Farenheit. In nature, the soil insulates the roots naturally and suitably adapted plants will survive cold weather. In my own Pennsylvania garden in zone 6, this means the tree may die back to the ground in a cold winter but the established trees will survive and resprout from the ground the following summer, however younger trees may be killed outright. I have also found that trees grown from a northern seed source seem to be more winter hardy than trees grown from a southern seed source. In any case, the tree is nominally hardy to warmer areas of USDA zone 6 but preferably warmer.
I think your best bet would be to plant it in a sunny spot, but in a sheltered location out of the wind and to apply a generous layer of mulch over the root zone but not touching the trunk itself. In your question you did not indicate if the plant has been kept outside up until now or if it is indoors. If it has been kept outside until now, plant it right away unless the ground is frozen. If the ground is frozen, store it in an unheated garage and keep the soil barely moist, then plant it outside in early spring when the soil is workable.
If the plant has been kept in a heated area and is still actively growing, you might be able to keep it growing over the winter if the location is bright enough. Otherwise, place it in a cooler location (above freezing but no more than say 55 degrees) with very bright light and allow it to rest, then plant outside in the spring after a gradual acclimation to the out of doors. Keep the soil barely moist while it is resting, but do not allow it to dry out completely. Good luck with your tree!
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