I've had a fitbit charge 4 for about three years now. I get my weekly report on tuesday. I hope that's because I first used it on a tuesday. (Planning to buy a new one and would wait using it if it indeed depends on the first day of usage). In my country, the week ends on Sunday and starts on Monday.
Best AnswerI get my reports on a Tuesday some time during the morning - I'm in Australia. I don't think it has anything to do with when I set up my first FitBit 10 years ago. I've been through 4 Fitbits in that time and have started them on various days.