I generally agree with your sentiment, but Twitter is a US company with most of their employees residing in the US (at least presumably?). So in this case it’s reasonable to assume they meant it’s a holiday in the US.
>>bombol+XK
As someone not from the US, it’s not a surprise at all. But if someone says a foreign company is making their employees work on a holiday weekend, I don’t fault them for not explaining what specific holiday it is.