e.g. Monday is - also according to ISO 8601 - the first day of week in the follwing countries🇩🇪 🇫🇷 🇬🇧 🇷🇺 🇪🇸 🇮🇹 🇵🇹 🇬🇷 🇸🇪 🇳🇴 🇩🇰 🇫🇮 🇳🇱 🇧🇪 🇦🇹 🇨🇭 🇮🇪 🇵🇱 🇨🇿 🇭🇺 🇸🇰 🇸🇮 🇭🇷 🇷🇸 🇧🇦 🇲🇪 🇧🇬 🇷🇴 🇹🇷 🇸🇾 🇯🇴 🇱🇧 🇮🇶 🇮🇷 🇸🇦 🇦🇪 🇶🇦 🇰🇼 🇧🇭 🇴🇲 🇾🇪 🇵🇰 🇮🇳 to name a few. 
best practice is to consider the current locale.