Organization expert Julie Morgenstern asked Peggy Post, an author who's carrying on the family business of defining polite behavior, how late is too late for the following scenarios?

A job interview
Any lateness is too much, if you actually want the job.

A business meeting or meal
Ditto. Your lateness wastes not just your time but the time of everyone else who's participating.

A dinner party
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 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.

Appointments with doctors, hairstylists, etc.
Don't expect to keep your appointment if you don't arrive on time. Call ahead and reschedule if you think you'll be more than five minutes late. Accommodating your tardiness could mean throwing off the doctor's or hairdresser's other appointments for the rest of day.

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.