With families now scattered across the globe, a reunion holiday is an increasingly popular choice. That same familial — and friend — diaspora also means people are taking much more time, often years, to plan for big gatherings to get all the pieces in place.  “People are realising that if they're going to invite 150 people to a big party, for the same money they can go away and celebrate with a tighter knit group of friends or extended family,” Dunn said, adding that grandparents often foot the bill for a multigenerational get-together.  But a once-in a lifetime destination celebration or milestone party takes careful financial planning. Consider that even a seven-day trip to Morocco could easily cost upwards of $10,000 for a party of 20 people, not including airfare and other incidentals. To make the most of a milestone celebration, it helps to have a plan.  Setting a realistic budget is your first task.  “You’ve got to start with the end goal in mind. What resources do you have and how are you going to save?,” said Michael Schweitzer, head of wealth sales and distribution at HSBC in the UK. “One of the biggest pitfalls is not being realistic about the time it takes to get there (financially).”  An online budget calculator can help you get started and simplify the process, and be sure to think of every possible cost you think you might incur.  “Most people tend to pamper themselves a little more during a trip of a lifetime, so remember to add in a buffer for treats and special experiences,” said Steven Ler, Senior Vice President of Singapore-based UOB Travel Planners, a subsidiary of United Overseas Bank.  Ask yourself whether you plan to treat others on the trip entirely? Will you pay for some things like meals or accommodation or special treats? Will you need to arrange transport or other details?  It’s vital to establish how much of the bill you will be footing and what others on the trip will pay for – and communicate this clearly to those you want in attendance.  For family milestone events, it is not uncommon for one person — or a few people (like grandparents or adult siblings) to cover the cost of accommodation and some meals and secure discount rates for excursions, but have attendees pay their own airfare and other incidentals.  Four years before the big event is the time to discuss your idea with family and close friends — and get anyone who might help plan and pay on board. You can brainstorm ideas and ensure everyone's input is included in the planning. Ensuring the key people are involved in the planning will avoid any disagreements closer to the event.  Research flights and hotels but also costs like taxis, restaurants, drinks and activities, to ensure the trip is within everyone's budget, not just your own. Organise your to-do lists and check dates. The party or trip might feel a little deflated if a key person is unable to make it as it out of their price range and they weren't given enough notice to allow them to save.  When Sarah Armstrong, 34, an interior designer based in France, decided, with her mother and brother, to throw a surprise party for her father’s 60th birthday, planning down to the smallest detail helped her stay within budget – about £6,000 ($10,000) for 100 guests.  She booked the venue about 18 months before the party and did not have to pay a deposit as she was a member of the Royal Air Force Club in London, where it was held. The club also provided caterers, meaning one fewer item on the to-do list.  Sarah Armstrong (third from right) and her mother (second from left) threw a bash for her father (third from left), staying on budget by planning ahead. (Courtesy Sarah Armstrong)  Sarah Armstrong (third from right) and her mother (second from left) threw a bash for her father (third from left), staying on budget by planning ahead. (Courtesy Sarah Armstrong)  At the time, Armstrong was living in the United Arab Emirates, so had to plan the party from a distance – she arranged everything except the live band, which her brother, a musician, organised.  She used Caxton FX, a currency conversion company, to transfer money from her account in the UAE to her UK account, waiting for a moment when the exchange rate was favourable. “If you use your bank overseas to send the money directly it’s highly likely you’ll be hit by charges at either end, plus an unfavourable exchange rate,” Armstrong warned.  Now is the time to really consider every budget item, including staff tips, which people tend to forget until last minute.  “Plan it very well so you don’t get any nasty surprises cost-wise,” Armstrong said. “Try to think back to an event you’ve been to and pick the elements that you definitely want, even down to details like place cards, extending bar hours or microphone hire – they all cost.” She also recommended including a 10-20% contingency fund for unforeseen costs.  Getting the most out of the money you’re saving can make a big difference, added HSBC’s Schweitzer. With four years to go, make sure you’re getting a good rate now that you’ve got a bit more saved.  “People think they can just keep their savings in cash but they are actually getting negative real returns as they don’t keep up with tax and inflation. They’re losing purchasing power,” said Schweitzer.  Keep an eye on your ideal venue, location or holiday destination. Are you still keen on the same place you were two years ago when you first started planning? If not, has somewhere better caught your eye? Is the cost comparable or do you need to adjust your budget?  Now is also the time to contact venues and suppliers and find out how far in advance you will need to book and what kind of deposit you will need. Many of the most popular venues are booked 18 months to two years in advance and require a deposit.  "Letting guests know an outline of your plans two-to-three years before, and estimating what you think it’s going to cost, is a good idea,” US-based financial educator and broadcaster Alvin Hall said. You’ve already told them about the trip, now is the time to outline the expenses and their part much more clearly.  Reassess and get savvy about putting your money to work. Channelling everyday spending through cash back websites or a credit card that rewards you with air miles also makes your money work harder, Lisa Conway-Hughes, founder of the misslolly.com financial planning website, added – but make sure you pay off the balance each month to avoid charges.  One Singapore-based couple charged their wedding expenses and business trips to their UOB PRVI Miles Platinum American Express Card and exchanged their miles for two return business class tickets to South Africa for their honeymoon, UOB Travel Planners said.  Don’t miss out on good offers on hotels, flights, travel insurance or car hire that may expire nearer the time.  “Consider booking your trip at a travel fair,” said Ler. “Singapore hosts the annual National Association of Travel Agents Singapore (NATAS) travel fair during which travel agencies offer great deals on travel packages.”  He added: “You could even pick up a new set of luggage bags as these are often given out as complimentary gifts during the fair.”  Start looking forward to your big celebration. Send out save-the-date emails or cards to remind your guests the countdown is on.  If you're flying to an exotic locale, schedule any inoculations you need and make arrangements if a visa is required. Consider writing a checklist for your guests to include with your 'save the date' to ensure that everyone is on track and made aware of any special requirements.  Now is also the time to tell your employer you'll be taking time off and to book your annual leave. Remind your family and other guests to do the same.  Focus on details that will free up spending money for treats: booking shared airport transfers for several people, for example.  “Booking a group dinner may cost less than just going to a restaurant – it’s about finding ways to make the trip more affordable,” said Hall.  If you're going abroad and will be spending a foreign currency, start researching the best exchange rates and currency offers for spending money, so you aren't stung by exorbitant rates or fees.  Finally, arrange any last minute minutia well ahead of time, such a writing place cards, song lists or menus. Then, with all your hard work done, you can focus on the fun and fabulous elements of your soirée. You can plan special treats, surprises and outfits.  Checklist: One year to go  To comment on this story or anything else you have seen on BBC Capital, head over to our Facebook page or message us on Twitter.
Whether itâs a birthday bash in Bermuda or celebrating a golden anniversary in Greece, hereâs a step-by-step guide to make that milestone trip a reality.