Is the VisitPlan.day attribute is an ENUM, the same as in the mentioned Sampler demo? In your message.properties I don’t see any localization values for Day, so I can’t say if your sorter function is correct.
The sorter function must operate with actual values, e.g. in this demo you can see that messages.properties and messages_ru.properties have different values in the sorter function. Since you generate your days, then you need to generate a sorter function with proper values.