The only way to do it is by changing your international region in settings. Use UK for Monday to Sunday. Unfortunately, that will change all of your other settings.
For some reason, Apple will not give us this option. Ask them to add the feature. People, like me, have been asking for years. They still won't do it and it won't be anytime soon. If your American, enjoy your UK settings and screwed up phone numbers but at least your calendar will start Monday.
All of us think it's stupid but it's been four years of iOS and still we have this problem. Don't be looking for a fix anytime soon.
Thanks for the tip, Mactagg. Yes, it's infuriating.
My settings are for Singapore and we follow the British system, yet the Calendar still starts from Sunday which makes me wonder why Apple decided that the UK Calendar should start from Monday...
True, although there are plenty of good stuff about Apple, it seems that perhaps some middle manager somewhere has screwed certain features up such as Bluetooth which is horrible but that's for another story and thread...
I will still stick to Singapore settings as it affects my phone number settings.