“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.
I’d be happier getting this than one of the out-of-office messages that provides waayyy too much detail — “I’m at home nursing an unhappy stomach, hope to be in tomorrow, but meanwhile am resting and checking email in between bathroom runs,” etc.

But I also believe there’s meaningful power in the mundane cultural norms we set and practice. Email, for better or worse, makes up a large chunk of how knowledge workers communicate. So much of this communication is muddled by broken email habits and larger anxieties around performing productivity. We’re constantly nervous about asking too much of others or doing too little on behalf of our coworkers. But we’re also stuck in work patterns that force us to communicate constantly and normalize working and demanding things from colleagues at all hours.
The OOO: was there ever a less apt acronym? (Ooo? Ugh, more like.) It wouldn’t be so bad if it actually worked when it was your turn to set one up, but unless you happen to live in France, where a worker’s ‘right to disconnect’ is enshrined in law, the twin fears of missed opportunities and the mail mountain that’s piling up in your absence will likely keep you furtively glancing at your in-box.