Earlier this year, AT&T put itself in a similar situation when it posted on the anniversary of 9/11.
Both apologized, with varying levels of success, shortly after.
But, it's better to avoid the need to apologize in the first place. The way to that is simple: Don't post about tragic events, unless the event is yours to post about.
By all means, you should stay on top of what's going on in the news. In fact, you should include tragic anniversaries in your content calendar so you know when they are. But those dates should not be mistaken for post inspiration. They're in there to remind you that in addition to not posting about the tragedies, you should thoroughly vet anything else you post that day in the context of the tragedy. Emotions will be high, so be extra careful. If your target audience was directly affected by a tragedy, you may just want to not post that day.
The only time you get to post about tragedies is if they are directly related to your business. For example, APEGA, The University of Alberta's Engineering Employment Centre and WISEST successfully posted this week about the anniversary of the École Polytechnique shooting, where Marc Lépine killed 14 women in his engineering class, because "feminists" had ruined his life.
Those organizations got a say because they're, respectively, the professional association for engineers in Alberta, an engineering faculty, and an organization that encourages diversity in engineering and the sciences. No one took the opportunity to promote themselves. As a result, they didn't get criticized, they got retweeted.
When we talk about the tragedies that directly matter to our organizations and our audiences while keeping it respectful, things are much less likely to go wrong.
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