Q.setting out transplants
When can I set out cabbage and brussels in the Denver area? Also, will hardening off be any different than summer crops?

You can set your plants out after danger of frost is past. You will need to harden off your plants.
Here are several links to help you.
https://www.gardeningknowhow.com/garden-how-to/seeds/how-to-harden-off-your-seedlings.htm
https://www.gardeningknowhow.com/edible/vegetables/brussels-sprouts/how-to-grow-brussel-sprouts.htm
https://www.gardeningknowhow.com/edible/vegetables/cabbage/growing-cabbage.htm