Formal Occasions :e.g. weddings, funerals, graduations, formals
Depending on the level of formality, wear a suit (more formal) or blazer/dress-pant combination. A tuxedo is even more formal. If you are unsure if the event requires formal clothing, it is always better to overdress rather than under dress.
You can't go wrong with a white collared dress shirt. Neutral colored dress shirts (grays, blues) work as well.
This is one of the rare times when tucking in your shirt is both attractive and necessary.
You don't want your tucked in shirt to be too tight, or too loose and baggy. To obtain perfect shirt bagginess, use this algorithm: tuck your shirt in, raise your arms vertical, then bend over and rotate about your torso with your hands outstretched. Your shirt now has the proper amount of bagginess in it.
If you're looking like a million bucks in your suit, don't tarnish your look with a sub-par coat. Our guide to outerwear is here.
More tips found in Shoes, Pants, and especially Personal Grooming
More event-specific tips: Beach, First Date and Clubbing
[Events]
"Clothes make the man.
Naked people have little or no influence on society."
Mark Twain