An automatic response should include the date when you left, a reason (vacation, sick leave, a conference, etc) and, if possible, the date when you plan to come back. Thank your client for their email, promise to get back to it as soon as possible and apologize for the inconvenience. Also, it should be obvious that you’re out of office from the first sentence.
Here are some examples of effective text for out of office messages you can use to keep your clients notified of your absence and unavailability during a holiday.

This is true! The nuclear option also helps the recently returned vacationer understand what is a priority and what isn’t. But, as boyd wrote, “if you just turn off your email with no warning, you're bound to piss off your friends, family, colleagues, and clients.” The blog post offers some helpful steps to make a clean break feasible — they include communicating with colleagues about the sabbatical long in advance, managing expectations of those who rely on you, creating a backdoor for true emergencies, and then, right before going away, reminding everyone about the sabbatical once again.
Sample voicemail message for office or department: “Thank you for calling The University of Toledo’s (office/department). The University is closed for winter break. Please call back after New Year’s Day or visit our website at (website URL). Thank you and happy holidays.”