I just notice the iOS holiday calendar on my iPhone (running iOS 11.4) becomes "Untitled Calendar".
According to this post, the holiday calendar is a part of iOS so there is no way to remove this calendar and then re-add it. It appears that this problem has existed since iOS 10.3.1 (https://discussions.apple.com/thread/7915337).
Do anybody encounter this problem? Any suggestion on a solution or workaround?