What to find out in advance: Will you be attending a house of worship with a certain dress code? Will you exchange gifts? Who drinks and who doesn’t? Will the adults ever go out alone, or will it be all family all the time?
What to give:

Get your mother-in-law a florist’s gift certificate so she can select her own arrangement, says Robyn Freedman Spizman, author of the gift-giving handbook Make It Memorable (St. Martin’s Griffin, $12 on www.barnesandnoble.com).

Treat your father-in-law to an old-fashioned shave at his barbershop.

Bring a stack of holiday classics: A Christmas Carol, “The Gift of the Magi,” “A Child’s Christmas in Wales.”

What to do:

If you’re new to the family, “go as an anthropologist and ask them about their traditions,” says Dallas marriage and family therapist Delane Kinney. While you don’t want to impose on your hosts, sharing a favorite recipe or tradition of your own can comfort you without undermining their customs.

Allow your spouse some time alone with his family — it’s decompression time for you.

Coming and going: Follow the same rules you would with your own family (see Week With the Family Guest Etiquette), disrupting your hosts’ schedules as little as possible.
How to follow up: Send a thank-you note with a gift of a gourmet cake or candy, suggests Spizman. Your in-laws can enjoy this themselves or serve it to guests if they’re doing more entertaining.