A sixth birthday, a fortieth,
and almost nothing in common.

The word birthday covers two events with almost nothing in common. One is a room of small children on a Saturday afternoon, where the risk is that it runs on too long and ends in tears. The other is a milestone, a thirtieth or a fortieth or a fiftieth, where the risks are a venue's minimum spend, a guest list spanning three generations, and half the people you want there needing a babysitter before they can answer at all.

So this guide keeps them apart. The next three sections are the children's party: how long it runs, how many children is manageable, the class-invitation problem, and the running order that stops a room unravelling. The three after that are the adult milestone: the venue and the money, the gifts, and the surprise. The last section belongs to both, because both end in a number you have to trust.

If you are planning one of each this year, read them separately. The instincts that make a children's party work, short and structured and finished before anyone is tired, are precisely the wrong instincts for an evening built around a long table.

Length first, because it is most often got wrong and always wrong in the same direction. Three- and four-year-olds manage about an hour and a half. Five to seven, two hours. Eight to ten, two to two and a half. Past that you are into cinema trips and sleepovers and different rules. Whatever you settle on, take half an hour off the number you first imagined: the common failure is a party that outlasts the children's attention, and the last twenty minutes of one that has gone on too long undo the first hour.

Numbers next. The rule people pass around is one child per year of age, five guests for a five-year-old, and like most rules of that shape it is too neat to follow literally and too sensible to ignore. The real limit is supervision, and one adult can comfortably keep track of four or five small children. Ten children with two adults is a party. Twenty children with two adults is a job. When the number climbs, what you need is more adults, not more games.

Then the class question, which is not really about logistics. In many places the convention is that you invite the whole class or you invite quietly, a handful of friends contacted directly with nothing handed out at the school gate. What causes upset is the middle version: eighteen out of twenty-two, arranged in the playground, so the four who were not asked find out by standing nearby. Choose one end or the other, and if it is the quiet version, send the invitation to the parents rather than through the children.

Small children do not need entertaining for the whole party. They need a shape, and it is nearly always the same: arrive and settle, run about, eat, cake, home. Twenty minutes of arrivals while people trickle in, forty-five minutes to an hour of the loud part, twenty minutes at the table, ten for the cake and the singing, then coats.

The order matters more than the content. Food before running about produces exactly what you would expect. Cake at the beginning throws away the only reliable ending you have, because cake is the signal that a party is over and children understand it without being told. Keep one thing in reserve for the twenty minutes when the weather turns or the entertainer is late: a story, a film, a box of something to build.

Put the shape on the invitation in plain words. "Two until half past three, food at half past two, cake just before the end" tells a parent everything at once: whether to feed their child first, when to come back, and how long the whole thing lasts. It is the most useful sentence on the page, and the one that prevents most of the messages.

Two questions decide how the day actually feels, and both belong on the invitation rather than in a week of messages.

The first is whether parents stay. For under-fours most of them do, and you are hosting two parties at once, so plan for adults in the room and put coffee somewhere. From around five, drop-off becomes normal; by seven or eight it is assumed. Whichever you want, write it down: "Parents very welcome to stay", or "Drop-off, collection at half past three", or, if you honestly do not mind, "Stay or drop off, whichever suits". A parent who is not told will ask, and one who does not ask will guess.

The second is what a child cannot eat. Allergies are not something to discover at the table with fifteen children already sitting down. They belong with the reply, in writing, from the person who knows, and that means the parent rather than the child. Ask in the same breath as the yes. A reply form that takes dietary notes and allergens alongside the answer hands you one list to read before you shop, instead of nine messages to search through on the morning. That is the whole argument for an online RSVP over a show of hands in a group chat.

Two smaller things are worth collecting while you have their attention: a mobile number for whoever is collecting, and whether the child is going home with someone other than the parent who replied. Neither matters until the afternoon it does.

A thirtieth, fortieth, fiftieth or sixtieth is a different animal, and almost nothing from the children's version transfers except the value of an order to the evening.

The venue is the first change. Past a kitchen and a garden you are into rooms that want a deposit and a minimum spend, a figure you commit to whether or not the room fills. That number, rather than the guest list, is what really sets the size of the party, so ask before you fall for a room: what the minimum is, whether it counts food and drink or only one of them, what happens if you come in twenty people short, and when the final number is due.

The guest list is the second. A milestone spans three generations in a way almost no other party does: friends from work, friends from school, a partner's friends, brothers and sisters, parents, and the aunt who will leave at nine. They want different things from the night, which is why a milestone needs a shape, arrival drinks, then a sit-down, then the part where the music gets louder, so the people who came for a conversation get one before the people who came to dance take the room. If only part of the list is invited to the sit-down, ask for the reply part by part, so a yes to the evening is never mistaken for a yes to the table.

The third change is time, and it is the one people underestimate. Much of a fortieth's guest list has small children, and for them a night out is not a decision but a negotiation: a babysitter, or grandparents, or a partner staying home. That takes weeks. Six to eight weeks ahead is a sensible minimum, and three months if there is travel or a night away. An invitation that lands a fortnight before gets warm regrets from exactly the friends you most wanted in the room.

This is the most common complaint about milestone parties, and it is never really about money. It is about finding out at the bar. A guest who knew in advance that drinks were their own arrives with a card in their pocket and thinks nothing of it. The same guest, told nothing, arrives as a guest in the ordinary sense of the word, and the moment they are asked to pay the evening quietly changes shape for them.

So say it on the invitation, once, plainly, without apologising. "Drinks from the bar" is a complete sentence and nobody objects to reading it. So is "Dinner and the first drink are on us, the bar is open afterwards". If the venue charges a set price per head and you want people to contribute, that is a harder message, and it goes out early and privately rather than on the invitation. Someone can decline an email in March in a way they cannot decline a request made in a room in June.

The same honesty applies to gifts. "No gifts" is well meant and almost never obeyed, because it leaves a guest with a problem and no solution: they still want to arrive holding something, and now they have to invent it themselves. The version that works is specific. Ask for one thing instead, and name it: a contribution towards a shared present, a bottle for the table, a letter for a book someone is putting together, a donation to something the person actually cares about.

If it is a shared present, somebody other than the birthday person has to run it, and the invitation should say who. A line as flat as "If you would like to put something towards a present, message Sara" does the whole job.

A surprise party has more ways to fail than an ordinary one, and nearly all of them come down to two things that get left to chance.

The first: one named person is responsible for getting the guest of honour to the right place at the right time, and it cannot be you, because you will be at the venue. That person needs a reason for the outing that survives being questioned, a plan for what to do if it is refused, and your number in their phone. Everything else about a surprise can go slightly wrong and be absorbed. This cannot.

The second: tell the guests a time thirty minutes earlier than the real one. Guests run late, and they run especially late to a party thrown by someone they have never met. Half an hour of margin turns a room that is half full at the moment of arrival into a room that is full. Put the request for secrecy at the top of the invitation rather than the bottom, since "this is a surprise, please do not mention it to Daniel" is only useful if it is read before the message gets forwarded.

Both parties end in the same place: a number. The caterer wants it, the venue wants it, the cake is sized by it, and the number of tables is a division sum. Almost everything you are asked in the final week is downstream of how many people are coming, and a guess is expensive both ways. Too high and you pay for empty chairs; too low and there is not enough of anything.

Which is why the reply is worth more than it looks. Ask for it in a way that produces a total rather than a conversation: a yes, how many people that yes covers, and the one or two other things you need, the allergy, the meal choice, the name of the child. Our RSVP deadline guide has the dates to use for different sizes of party, and the birthday invitation wording page has the lines themselves, including the reply line and how to word a surprise. Set a date, put it on the invitation, and treat the week after it as chasing time rather than bad news. People rarely decline; they forget.

One last argument for an invitation that lives at a web address rather than in a photograph: things change. A start time moves, a room number turns out to be wrong, the pub floods on the Thursday. A link is corrected once and every guest sees the new version, including the ones who opened it a fortnight ago. A picture sent to forty people cannot be recalled, and the correction always reaches fewer of them than the mistake did.