A whole new use for straight quotes

It isn't emphasis. The quotes make it clear that there is controversy over whether or not it is St. Patrick's day. It also separates the author/editor from the "not" so that he/she isn't expressing an opinion. This is a standard use of quotes– and it doesn't really matter if they are single or double. Single quotes are much more common in the UK than the US. The BBC is also consistent with this usage of single quotes in headlines.

Maybe. I agree that in most other stories on their site, the use of quotes is correct. I’m not so sure about this one; I think if you wanted to indicate that the opinion was controversial and not necessarily the author’s opinion, you would write

Why is it “not St. Patrick’s Day” today?

since that’s the controversial claim, surely?

Try reading it out loud; I’m pretty certain that the quotes are—at the very least—in the wrong place, and where they are at the moment, it sounds like emphasis when you read it out. Personally I wouldn’t have used them, since it’s clear from the story that the author isn’t saying that it isn’t St. Patrick’s day, just pointing out that some people make that claim. But if I was going to use them, I don’t think I’d have put them just around the “not”.