If it were me, I'd ignore the "holiday casual" clause and go with the tradition from past years. Stick with the same level of dress that you mentioned you've worn before, but with a kilt.

I would be tempted to wear a traditional tartan wool kilt, day sporran or non-formal evening sporran, proper hose (not scrunched down) with dress shoes (not formal, but not casual), a shirt and tie, and a tweed jacket. Sort of a nicely dressed look, but not approaching formal by any means.

I'm assuming this is an evening party, held indoors? What's the venue? Is it held at the workplace, or at a hotel or convention center, etc? Is it dinner and entertainment, dancing, what?