Gentlemen, you need to own at least one suit in order to be a grown-up.

The editors at The Morning News have already done a good job of explaining why this is so, so I'll quote them:

Without suits, men would have nothing. In the hierarchy of style, a good suit remains a man's only trump card. Even in this sad age of casual-wear, the suit still carries an air of success, taste, and sophistication. It is designed to make you look better, to break boundaries between social classes, to make a small man tall with pinstripes or a fat man rich with soft wools. The suit looks good in restaurants, trains, dinner parties or Paris; in short, everywhere you want to be. It is, in its best forms, a complete outfit that will never fail you.

If you own only one suit, I recommend going with a basic black suit with a classic cut. Like the Little Black Dress, that staple of women's fashion concocted by none other than Coco Chanel herself, everyone looks better in one, regardless of their shape, size, colouring or goofiness. It fits the widest variety of occasions, from business meeting to cocktail party to heavy date to funeral (even your own). With a crisp white shirt and a good tie (and remember, the basic-ness of the black suit will allow your to express your personality with your tie, from reserved and dignified to fonk-ay accordion player), you will never be turned away from any semi-formal or formal event. You can funkify it further with a coloured shirt and go tie-free when restaurant-going and martini-drinking (where you can even pull your shirt collar over the lapels if you wish), and a banded-collar shirt makes the black suit suitable for clubbing. If the black suit is good enough for the Yakuza -- the Japanese mafia, simultaneously the most stylish and deadly men in the world -- it's good enough for you.

Here comes superficial, old-fashioned and judgemental statement number two:

If you are attending a wedding, you must wear a suit.

Why? Because weddings are special:

• They're one of those rare major changes in your life that are also voluntary. You have no choice about getting born, coming of age or dying (suicide or personal sacrifice notwithstanding), but in Western culture -- shotgun weddings aside -- you choose to get married.
• They're unique moments. Ideally, you'll have only one in your life. Some people might have two, and Liz Taylor, practitioner of Extreme Matrimony, hasn't been able to do more than eight (and twice to the same man).
• Regardless of culture, they are considered to be so important that only someone in authority -- a justice of the peace, a holy man or woman, a captain of a ship, the owner ot the casino -- can perform the ceremony.
• In most cases, the bride and groom are covering the cost of dinner, if not the drinks.

As a guest, you have been asked to share this important, life-changing, expensive moment, and to be respectful, you should dress accordingly. This is a special occasion and it's no time to be wearing khakis and that slightly-more-expensive shirt from the Gap.

There are exceptions to the suit rule for weddings, and they are:

• If the culture in which the wedding takes place has some other standard outfit equivalent to a suit. In my own home country of the Philippines, which gets extremely hot, a barong tagalog (a light embroidered shirt worn untucked over black dress pants) can be worn to even the most formal events. In India, a kurta payjama (the article of clothing from which we derive the English word for sleepwear) is as good as a suit. If it's going to be a Dungeons and Dragons wedding, make sure you wear your dress armour and the nice medallion your character Galstaff took from the orcs he defeated.
  
  
  
  Formal wear, Filipino style. That's my cousin Manny and me in our barongs, getting ready to do the readings for our cousin Rowena's wedding in Manila. The barong is made out of fibers made from pineapple husks, which makes it a very light material that keeps you cool in a country with serious heat and humidity.
  
  
• If the invitation says that the wedding is informal and takes place during the day. This may also apply if the wedding is taking place beside the line for people who are getting their driver's licenses renewed.
• If the officiator is an Elvis impersonator and will be handing the bride and groom some complimentary chips for the casino at the end of the ceremony
• If the father of the bride needs to keep a Winchester trained on the groom throughout the ceremony
• If the ceremony is taking place in the shower room and the "bride" (who may be referred to using more colourful terms) is emitting muffled screams along the lines of "for the love of God someone please call the guards".

Recommended Reading

The Morning News' editorials on dressing up. In this series, they cover suits, dress shirts, pants and then wrap it up nicely.

So You Wanna.com's piece on buying a suit.

The horrible, horrible things that happen when men wear the Little Black Dress.