I bought a new acrylic free standing tub. It was delivered and is sitting in the garage until my husband gets home from out of town. Do I need to be concerned about the freezing temps out outside? It's in the 20's at night and 40's during the day. I'm wondering if it would crack or anything? What are your thoughts?

Not unless you drop it or fill it with water out there.

Possibly you should be VERY concerned. Spa manufacturers water test their spas to be sure there's no leaks. Rarely if ever do they properly drain all the water out (of all the plumbing and equipment) in anticipation of it sitting in freezing conditions. If it gets below freezing in your garage, you could have some serious damage occur to the spa.