I was browsing trough the hot questions and I've stumbled across this question. One of the comments mentioned checking the date, but the bells didn't ring until I read the accepted answer.
Regarding the site from the question, it isn't stated anywhere that it is a prank, so it could be just a coincidence.
In my opinion this isn't a good user experience as the user may not know about this day, as it is not a public holiday as stated by Wikipedia, or they could just be busy and forget about it. In fact maybe they don't like jokes, because they are a mature person who just wants to get their work done, or maybe they are a robot.