What to find out in advance: What is the suggested attire? Are kids invited? “Settle any doubts ahead of time,” says event planner Kimberly Schlegel, author of The Pleasure of Your Company (Gibbs Smith, $23 on www.amazon.com).
What to give:

Not flowers. “They may not go with the look of the party, but the host would feel obligated to put them out,” explains Schlegel. Instead, try a low-maintenance potted plant, which will last beyond the holiday season.

A pail full of shiny red apples or a box of small, sweet clementines.

What to do:

RSVP. Even for a casual event, the host needs to plan how much food and drink to get, says Schlegel.

Make an effort to mingle, so the host doesn’t feel she has to entertain you.

Don’t take the term “open house” literally. Keep to the areas clearly decorated for the party.

How to dress: A crowded party can get hot. To stay comfortable, make sure the layer closest to your skin is something lightweight. A colorful shawl adds festive flair and is easier than a sweater to put on and take off as necessary.
Coming and going: Showing up anytime from five minutes after the designated start time to 30 minutes before the stated finish is OK.
How to follow up: Send a thank-you note, and if you have talked to the host about something in particular — a DVD she might like, for instance — send that along, too.