Samsung
Set the First Day of the Week in Calendar auf Samsung
Samsung Calendar inherits its week start from the standard location configured on your phone, not from a fixed calendar standard. That is why devices set to the USA, Israel, Japan or China often open on Sunday, while most European setups default to Monday. If you changed your region, restored a backup, or set up the phone new, this location-based default can override what you actually expect to see.
The setting only affects how Calendar lays out weeks and week numbers. It does not touch appointments, reminders, or synced accounts, so switching it is safe at any time.
To change it, open Calendar ➔ Open the Menu ➔ Tap on Settings ➔ Tap on First day of the week ➔ Choose an option. Pick Saturday, Sunday, Monday, or Locale default to match your preference.
If Calendar still shows the wrong day after this change, check whether a synced account, like Google, is overriding the local display, since some accounts apply their own week-start logic on top of the device setting.