Regional customs vary, from being on time to arriving 15, or even 30, minutes late. (And it's rude to arrive early; you might surprise the host and find him in his bathrobe vacuuming.)
A restaurant date
Even if your lunch or dinner mate is easygoing, it's disrespectful to arrive more than five minutes late.
A movie or play with friends or a date
Aim to arrive at least five or ten minutes before the curtain goes up. Arriving after showtime can spoil the whole evening.
A wedding ceremony
Arrive at least 10 to 15 minutes early. No one came to see you—a guest—walk down the aisle. If you arrive after the bride's and groom's mothers have been seated, keep a low profile by using a side aisle and sitting in the back. If you're so late that you fear you'll be elbowing the bride as she prepares to make her entrance, wait outside until her processional is under way.
A cocktail party or large reception
A delay of 10 to 15 minutes (sometimes 30 minutes or even more) is fine for fluid gatherings where people are invited to come and go.