You are grouping together two extremely diverse cultures. "Western" culture and "Japanese" culture are two completely different things -- and sometimes more than two, considering how "western" implies a great many cultures, and many of those holidays are celebrated across Earth in many different dimensions. But you are treating them like they are so close as to be a negligible difference.
As I said, the significant majority of the prison celebrates the holidays of one of those two cultures. When the chronological matrix indicates a significant number of simultaneous celebrations of a holiday, that holiday is dismissed unless there are inmates who would be interested in it. Why celebrate Heaven's Day or Peon Day if there is no one who would know what that is or why it is significant?