If we reject the theory that it could be someone’s elaborate revenge, Saturday may be the first day of the week that may become workday or non-workday because of incorrect assumption about the first day of the week. If everywhere but one place in your software the day numeration is correct it would be a hard bug to spot.
Also, if it is in Java, I vaguely remember there being a lot of ways to express weekday, so a lot of ways to shoot off your foot (solely on Saturday)