Currently, the first day of the week in a calendar depends on the user's language setting.
https://support.atlassian.com/trello/docs/turning-on-international-timedate-formatting/

Setting your account language to English (UK) will cause Trello to show Monday as the first day of the week, show dates as DD/MM/YYYY, and use 24-hour time.

However, for example, some Japanese users are accustomed to a calendar that starts on Monday. In this case, they are forced to use Trello in English, which could lead to reduced productivity. It would be beneficial to have an option to select the first day of the week in the calendar, regardless of the user's language settings.