What to wear?

I’m celebrating Christmas <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">Eve</a> with my boyfriend’s parents and his god family. I asked him what the dress attire is and he says classy/casual. I don’t want to wear denim to it, but it’s too cold to wear leggings. I’ve grown up wearing nothing but denim, so I am comfortable wearing them but yeah .. idk what to wear.