Questions About the Calendar in "Bible Black"

Figured I'd take my extremely niche question here.

So, in Bible Black, the 26th of April is a school day on Saturday. As the game was released in 2000, this appears to be completely normal given the fact that Saturdays were school days in Japan up until 2002.

However, according to this weird site I found, the second and fourth Saturdays of the month shouldn't be school days. A throwaway line about the end of the century in the game implies that it is set in 1999 (which also puts its backstory in 1987). Lo and behold, the 26th of April 1999 is a Saturday... the fourth Saturday of the month, in fact.

Is this just a plot hole or is there something I'm missing about the transition to off-Saturdays?