I’d just stick with “I will be OOO without access to phone or email from XX/YY to XX/YY, returning on XX/YY.” And then whatever directions for directing to your support/backup while you are out. I find that specifically saying ‘without access to phone or email’ sets a good expectation of non-response.
If your message requires a response faster than that, please email my manager at [email protected].

The appropriate tone depends on the holiday. If you’re closing for Christmas or New Year’s, it’s safe to assume your customers expect you to be festive and maybe even humorous. Conversely, if you’re closed for Veterans’ day, your email should strike a more serious chord. Use our templates and double-check your tone before you hit send.
Over time, I began to suspect that those who might be telling the truth (of which I suspected there were very few), would mention their relationship to the person and possibly the cause, for example “My grandmother just passed away after surgery.” The ones who left it wide open (“someone close has recently died”) led me to imagine that it could be a random person in their city that they read about in the news, their goldfish, or perhaps a distant relative who had passed away in the previous few years.