I would take it in already if that is your forecast. Is this a one night event or are extended nights of frost being predicted?--That makes a BIG difference.

Thanks, njoasis. It's just a one day event for at least the next several days. I did bring them in. Maybe I'll put them outside tomorrow again. I had signed up for one of the local news stations to send me frost alerts by text message.

Bob, one or two nights won't hurt your Citrus, but even though they'll be warm days, you should consider leaving them indoors.

Seems to me, the colder it gets, once a Citrus is brought inside, they tend to drop leaves more than a Citrus set indoors before cold. It happened to my Citrus.
Leaf drop worsens when Citrus are placed in rooms at 72F degrees or higher.

If the room your Citrus will be wintered is cooler, there's less chance of leaf drop.

"Is frost worse for the tree in spring or fall?"

Probably neither season, but of the two, I'd say fall. During spring, usually temps increase as time goes by, so eventually a Citrus tree adapts.
During autumn, 'at least here in IL,' temps drop dramatically. 80F one day, 55F or less the next.
Just my opinion...Toni

38'F and a chance of frost is not bad for citrus. if there is new growth, i would bring it in, but further up north, your days have been shorter and cooler than ours down here. so your trees should already be adapting to the autumnal season change. my trees put out new growth a few weeks ago and are starting to harden off.

on that note, i would say spring is worse for frost dmg b/c of tender new growth. it would only take one night of frost to damage new growth.