“The Rules” is a Moneyish series where we define the rules around sticky money or workplace topics.

Even an 8,000-mile distance might not be enough to get you out of an obligatory trip.

When travel writer Steve Long, 29, promised an old coworker that he’d attend his wedding this July, Long didn’t realize he would end up moving from Toronto to Chaing Mai for work. In all that drama, “I completely forgot about the wedding,” he says.

But about a month before the wedding, the friend reached out to Long to ask if he was still planning to attend. “As he sent that message I was like ‘oh f***. I didn’t want him to feel like I’m not valuing this friendship,” he says. “I made a split-second decision, I wrote back, ‘Oh yeah, of course I’m coming.’”

But Long says they were never that close of friends — and the trip cost him a lot. When he looked for flights, the cheapest one was $2,000. “That was just way too much,” he says. So “I ended up spending 100,000 points — all my points, which I had actually hoped to use for my vacation this year.”

Welcome to the era of the oblication. One in three Americans say they’ve taken at least one vacation where they felt obligated to do so, according to a survey from travel firm Travelhorizons — be it because of friend or family expectations, holiday traditions or because of the significance of the event like weddings, graduations, baby showers, etc. And less than half (46%) say that they enjoy these vacations as much as they do vacations they plan for themselves.

And these oblications are costly. Data from Hotwire, as reported by ABC News, found that people spend as much as $197 billion a year on trips that they feel obligated to take. And more than nine in 10 people use their vacation time from work to take these trips.

So Moneyish asked experts the rules around attending — and declining — a trip you don’t want to go on but feel obligated to take.

Know which oblications are required attendance — and which you can blow off. As a general rule of thumb, “you should try not to decline landmark oblications that can’t be repeated. For instance, your father’s 90th birthday party. Or your sister’s destination wedding. Or your first annual company retreat. These are oblications that are hard to do over,” says relationship and etiquette expert April Masini.

Some must-dos: “If you are in the wedding party you need to attend all functions/parties. Are you a godparent? A must. Bosses housewarming/dinner party: very good idea,” says Constance Hoffman, founder of Social & Business Graces. And Pamela Eyring, the president of the Protocol School of Washington, says that you should probably also attend many family obligations. “It could be worth the investment of time to attend by seeing family members who miss you or are aging,” she says.

Meanwhile, says Masini, “decline regular oblications. If your sister is on her fifth wedding, and it’s a destination event, you don’t need to feel the same obligation as you would if it were her first or even her second … And if your father is turning 67, and it’s a vacation birthday party that he has annually, you’re off the hook. You can attend his 68th.” Hoffman notes that you can “feel free to skip co-workers jewelry parties; showers of distant, non-close relatives; office’s happy-hour. These events are most defiantly optional.”

If you aren’t sure if you should attend, buy yourself some time. “Never feel obligated to answer on the spot,” says Hoffman. “This way you are less likely to have a cancel later. If you are put on the spot to offer a reply, just state you need to check your calendar/significant other, kid’s schedule, etc.” Eyring notes that: “Saying no is not as a problem as saying yes and then canceling at the last minute!.”

Say no politely but firmly. First, thank the person for the invitation. Then offer an explanation for why you can’t attend, says Masini: “You don’t have to go into TMI, but you do have offer a reason. You can say that you’ve got a family medical issue that is going to keep you from attending, or you can simply say that it’s not a year when you can afford this trip,” she says.

If the person seems upset, “don’t yield under pressure or tears. Instead, listen closely to his or her objections. Then, gently but firmly, restate your no and keep it simple and direct; no backpedaling or details,” says Hoffman. “Make statements like, ‘Not now’ or ‘I have another commitment’. How you deliver the news is more important than the words themselves. Sound and look grateful, however don’t go overboard.”

Eyring gives this example — of a board invite you want to decline — to help you learn how to push back: “Ann, I appreciate your invitation for me to join the board of advisors. When I accept to be on a board or have membership with an association, I commit to being active. There’s nothing worse to me than saying I will participant and then have to cancel. My business travels will be very unpredictable these days in my new position. I will not be able to accept your kind offer.”