You know that I am in habit of giving you a quick reply but this time I am facing difficulty due to the reason that I have to go to attend an annual session which is mandatory for my efficient official working. Therefore, I cannot reply to you on time. I hereby submit my apology for that. After I come back from attending the session, I will respond to all of your emails within a few days, most probably I will join you on 6-01-20XX.
I set my OoO for individual public holidays, because I know they’re not global, and I get emails from lots of different countries.

I’m with you on this one. Management has access to a mansion and a townhouse in two different fabulous vacation destinations and it burns my butt every time I see an out of office from one of them (98% white men) going on about how they’ll be enjoying this perk. In the meantime, a few years back we had to eliminate free coffee at the offices because business was not good enough (it was eventually brought back after company president realized after a year that people were really pissed).
“No matter what your best intentions, you're going to get bogged down the minute you return to the office,” Sullivan says. “Don't set exact timeframes for responding to messages in your OOO. Instead, set general expectations for‘I'll respond as quickly as possible upon my return,’ giving you some leeway.”