I had coworkers (and people up my chain of command) that did/do this. The thing all those people had/have in common is that they came from the military. So I always took it as a military thing. Curious if that particular coworker had a military background.
What are holiday out of office messages? Holiday out of office replies are automated email messages that professionals use to let others know they cannot respond due to being on vacation during a holiday. These messages typically include: An apology for the inconvenience

I used to work with someone who had a message telling people she only checked her email twice a day. You pretty much needed to call her if you needed anything outside of those times. (She worked in a remote office.) I think she had read one of those books on efficiency that recommended scheduled email time. But there were problems with this: 4. My department often had to email attachments or text to illustrate our questions/concerns. And we were on deadlines. Reading a page of text over the phone was not an efficient use of anyone’s time 5. She did outreach & was often out of the office on site visits, trainings, or travel to these places, but never ever set her OOO for these, because she was “working.” However, she was effectively not available to read emails from other staff until after hours on those days.
If you want to add a humorous spin to your vacation responder email, here’s a great idea: