Cocktail parties are always a lot of fun and relaxing events. That said, the formal name of this party insinuates a need for dressier clothing. For women this often means a cocktail dress, but for men it simply means business casual clothing.
Considering women will often wear a dress to a cocktail party, men should match suit and wear more formal clothing. A suite or tuxedo could be appropriate for a very formal event, but typically business casual attire is appropriate. This includes dark wool pants and a long sleeved shirt for men and often a sport coat. Lighter color pants and chinos may be appropriate for some parties that occur outside, during the day or during the summer.
When in doubt, wear a sport coat. You can always take it off later
You'll probably be standing a lot, so wear comfortable shoes
Finger foods are easy to spill on your clothes, so be careful