When the office Christmas party invitation state black tie (semi-formal) which is only held in the evening, it means tuxedo for men and evening gown or dressy cocktail dress for the ladies.
For casual office Christmas party, the men can wear a suit with or without tie and for the ladies informal attire like knee-length dress or a blouse and a slack.
For lunch office Christmas party, you’re most likely in your office attire. But maybe on that special day, you can wear a nice structured dress with coat during work and leave the coat behind for the party.
Then lastly there’s the office Christmas party at your boss’ house. Whether it’s a festive holiday dress code or a relaxed casual attire or cocktail attire, make sure to dress appropriately.