The Arabic/Islamic calendar starts at Sunday because the weekend is Friday and Saturday. In Islam, the weekly congregation (mass equivalent) takes place on Friday around noon.
Growing up in Saudi Arabia, we started our work week on Saturday, with Wednesday being the last day of the work (school for me) week. It took me a long time to transition over to think about Saturday/Sunday as the days off. I still get random pangs of excitement on Wednesday thinking it’s the end of the week if my brain hasn’t warmed up.
I believe this is still the case in the gulf states and I would assume other Islamic countries.
You made me think of Malaysia, Brunei, Indonesia: All are Islamic majority in Southeast Asia. I Googled and look what I found: "Identify the first day of the week based on the locale"
