When building global applications in Lightning Web Components (LWC), developers should never hardcode locale-specific logic. Salesforce provides a robust Internationalization (i18n) service that exposes the current user's regional settings directly to the JavaScript controller.9

The correct approach is to import the @salesforce/i18n module (Option D). This module contains several properties that are automatically populated based on the logged-in user's Salesforce profile settings. Specifically, the firstDayOfWeek property 10returns an integer (0 for Sunday, 1 for Monday, etc.) that corresponds to the cultural standard of the user's locale.

Using this built-in property is superior to other methods because:

It requires no Apex callouts or SOQL queries (unlike Option A and C).

It is automatically maintained by Salesforce; as global standards change, the platform updates the i18n values, ensuring the component remains compliant without developer intervention.

It avoids the maintenance overhead of creating custom metadata (Option B) to map every possible locale to a starting day.

By leveraging @salesforce/i18n/firstDayOfWeek, the developer ensures the calendar component is truly global and provides a native, localized experience for every user regardless of their country.