Punctuality is generally very important and a part of being a well-mannered and respectful person, but there are also some exceptions to this rule.
If you are going to dinner at a friend's house, it is generally OK to arrive about 15 to 20 minutes late. This of course depends on the friends and the occasion. If it's a dinner party with a number of guests, and you know that there will be drinks and hors d'oeuvres before the meal, it is generally acceptable to be fashionably late. In some cases, you may actually surprise your host if you arrive exactly on time, and do not arrive early unless you have arranged this with your hosts. Remember that they may be in their final preparations shortly before guests arrive, and it may be difficult for them to finish getting everything ready and entertain their guests before the appointed time (and they may even expect another 15 minutes to prepare).
If you are going to dinner where young children are involved, it is probably a good idea to be prompt (or just five or ten minutes late) as children have more regular mealtimes and bedtimes. If your dinner is before a workday, then again you will probably want to be just slightly late, as most people will not want to make it a late night.
If you are going to a party, it is generally acceptable to come half an hour late (or even more), but of course it depends on the type of party (for surprise parties, you of course must arrive on time). For large parties that are going on for hours where a meal is not served, it is generally acceptable to drop by for the portion of the party you like, as long as you don't arrive too late. If the party is being thrown by a close friend, however, you should plan to stay for the whole affair and even arrive on time to help get things started.
Remember that customs differ in different regions and that you must use your judgment depending on the situation and the people involved, and it never hurts to ask your host if you are unsure.
So here's hoping you and your guests always arrive at exactly the right time, and thank you for listening to quick and dirty tips for a more polite life.
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