When he tested it out by changing his location to the U.S., U.K. or Russia, the proper holidays showed up on the calendar app. Our "victim" not only sent Apple an email about what happened, he also put up a YouTube video, which you can find below.
Meanwhile, Apple sent him a response to his e-mail, which stated that there is a software issue with the calendar, which will be resolved with the next update. So, if you have experienced this issue for yourself, no need to panic. It will all be resolved with iOS 7.1.3, although no timeframe for that update was mentioned by Apple.
Calendar app has bug on iOS 7.1.2
Thanks for the tip!