Two different jobs behind one closed door
When you are closed, a visitor arrives with one of two needs, and they need completely different responses. Either something is wrong now and waiting is not an option, or something can wait and they want to know until when. Almost every out of hours message treats these as the same person.
The failure mode of merging them is specific. The urgent visitor reads a polite message about business hours and a form, decides you cannot help, and either goes elsewhere or has a worse outcome than they needed to. The non urgent visitor gets an emergency number they do not need and rings it, which costs the person on that number their evening.
So the first design decision out of hours is a fork, not a message. Establish which of the two situations this is, in one step, then send them down a genuinely different path. The fork can be as simple as one sentence naming the urgent route followed by a form for everything else, which is enough because urgent people read the first line and stop.
The emergency route has to be a route
An emergency route is a way for somebody to reach a human being now. A form is not a route. An email address is not a route. A page describing what constitutes an emergency is not a route. If the only thing behind your emergency wording is a slower version of the ordinary path, you do not have one, and you should not imply otherwise.
Businesses that genuinely have one usually already know what it is: an out of hours number, a rota, a partner service that covers your closed periods, or in regulated trades a statutory service that exists for exactly this. Index that, put it in the fixed wording rather than leaving the assistant to compose it, and check the number still works.
If you do not have an emergency route, say so plainly rather than dressing up the ordinary one. A visitor told that nobody is available until nine in the morning can make a decision. A visitor who believes a form will reach somebody tonight cannot.
The message form is a different thing, and should look like one
For everybody else, the honest framing of an out of hours form is that it is a way to be in the queue when the queue starts moving. That is genuinely useful, and it is more useful when it is described as what it is rather than as contacting the team.
Say when the queue starts moving. Not opening hours in the abstract, which the visitor has to translate into a time, but the actual next moment somebody will look. There is a real difference between we are closed now, open at nine, and somebody will read this from nine tomorrow morning. The second one answers the question the visitor is holding.
Then send a confirmation that repeats the same thing. The visitor closes the tab and forgets the wording within a minute, and a confirmation that repeats the time is the only artefact left. If the confirmation says something vaguer than the widget did, you have quietly downgraded your own promise.
Why shortly is the worst word in support
We will get back to you shortly does three bad things at once. It promises speed without committing to any, it means something different to the reader than to the writer, and it makes the eventual reply feel late no matter when it arrives. A visitor who reads shortly is imagining hours. A team that writes shortly usually means the next working day.
The word is popular because it feels warm and costs nothing to write. It costs plenty later, in the second message asking whether anybody saw the first one, and in the complaints that begin by quoting it back at you. Vague reassurance does not reduce anxiety, it moves the anxiety to a moment when you are not there to manage it.
Replace it with a time. Any specific time beats shortly, including a slow one. By the end of tomorrow is a better sentence than shortly even though it promises less, because it can be kept, and because a visitor can plan around it. The whole trick is that support expectations are judged against what was said, not against the clock.
Wording that survives Monday morning
Write the out of hours message so it is still true on the worst day. The worst day is usually Friday evening, and the sentence you need is one that does not become embarrassing when the reply lands seventy hours later.
A workable shape: state that nobody is available now, name the urgent route if one exists, say exactly when somebody will read this, and say what will happen when they do. Four short sentences. Nothing about how important the visitor is to you, which no one has ever found reassuring.
Check the wording against your actual calendar at least twice a year. Public holidays, seasonal closures and the week between Christmas and the new year all break a message written for an ordinary Tuesday. A business that closes for a fortnight in summer and says next working day throughout is generating complaints on purpose without meaning to.
The hours you actually keep, not the ones on the door
Published opening hours and support hours are frequently different, and the assistant usually knows only the first. A shop open until six whose inbox is monitored until four will tell a visitor at half past four that it is open, which is true and useless.
Index the support hours separately and say which is which. Visitors ask both questions and they are not the same question: whether they can walk in, and whether anybody will answer a message. Being able to answer both distinctly is one of the cheapest improvements available on most sites.
Then decide what happens in the gap. A message arriving at five past four is out of hours for support purposes even though the door is open, and the reply expectation should reflect that rather than the shop sign.