For a company sponsored cocktail or Christmas party, take your initial cues from your company’s dress policy.
If you work in a suit and tie environment, you can assume that suits will be worn at these types of social events. It is, however, recommended to switch from your normal solid white or blue dress shirt to something more fashionable. Try a striped dress shirt , check dress shirt, or a shirt with a contrasting white collar. For a more formal look, wear a French cuff shirt.
If your day-to-day environment is business casual, you may want to select a deep toned sport shirt and match it with a great looking sportcoat. Or, take that same sportcoat and wear it over a fine gauge knit.