If you’re just going to plant it, plant it now. If you’re going to play with the rootball after popping it out of the container, wait until spring. Sink the pot into the ground over winter if you’re not planting it now.

Sassafras should have solid hardiness for you...wouldn’t call it ‘iffy’ anywhere within 20k of Lake Ontario or west into SW Ontario.

I'm 40km from the lake!

Are sassafras juglone tolerant? And what type of a root system can I expect? Are they shallow roots? Is there a tap?

It's tolerant and the root system is a sparse far-ranging mat.

so not too deep? I have the edge of a septic field buried 18" to 20" about 20' from where I want to plant..