I have a colleague who usually does different ones for internal and external: internal will be “I’m currently in the pouring rain in a tent in the Cotswolds. I get back (hopefully without trenchfoot!) on Monday Date. Whilst I’m getting soggy please contact Email Address.” and external is just “I’m on annual leave between X and Y and will not be accessing my email. Please contact Email Address if you need assistance during this time.” We all really like their internal ones.
“Happy Holidays” is typically used when you aren't really sure what holiday someone celebrates. In that instance you are replacing Christmas or Hanakkuh with the word Holiday making it a proper noun, which means it should be capitalized.

A relatively unprofessional one — like mine, for instance — does the opposite: It encourages prospects, recruiters, and potential connections to run in the other direction.
With an out of office email, you provide the following information, preferably in a unique way: At the moment, you are not available;The exact time (date) of your return;Contact information (phone number) for urgent cases;Contact information of colleagues to be contacted in your absence;