I’ve seen a previous request to have the start days of the week with selectable or follow system settings.
Now that we’re up to version 9 I wondered if this is still an open issue to be addressed in a future update. For reference, I am living in Scotland and favour Monday as the start of the week.
First Day of Week is a per-user setting in macOS Sonoma in Settings > General > Language & Region.
But, I assume the OP has noticed that the date-picker in Tinderbox does not respect that setting, unlike other features in Tinderbox that do follow the system’s regional calendar settings.