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)
For bonus points, this failure is in a cron job that sends out recently queued messages. It runs once every ten minutes - last weekend we had 12 failures: four were in a cluster on their own, one was in a run of two, and six were in a single continuous run.
Please note that this server is unused by our business so no messages ever get naturally queued. Every day we sync the live production server to this server at about 9 PM - assuming an employee was queuing up a message before the snapshot is taken there might be a number of unsent messages in the snapshot - those messages will all be sent by the first cron job after the sync.
It is a wonderfully awful problem that has me wanting to pull out my luscious locks.
Yup, luck is appreciated and I'm trying to get more eyes but unfortunately I'm a senior dev that has the second highest seniority at the company so I feel guilty dragging others into it.