Basing from my experience, even if "formal attire" is indicated on the invitation, some guests would arrive in casual wear. Actually depende din yan on how you wear your clothes. On some formal occasions I would usually wear a short sleeved shirt covered by a formal jacket (blazer), a good pair of jeans or chinos (no jologs designs please), loafers, or brogues, and no neck tie. I get along just fine.