I think you got this wrong. If the highest return is on a Monday then you'll buy at the highest price on a Monday. You need to buy on a Wednesday, when the price tends to fall...then you'll lock in the best price and stand a better chance of realizing positive investment gains.
Maybe it's because Sunday is a holiday in most of the country, so people cash out crypto money on Sunday and that impact on Bitcoin price on Monday, so, when the price is low, that's the buying time without any doubt.
Why would people cash out their bitcoin on a Sunday? Why would it matter that Sunday is a day of rest for most people? I'm not following your logic.
With bitcoin and other cryptocurrencies the trend will change in no time. We can't experience the same pattern of market movements every week. Most of the time we can experience the price to increase at the weekends than during the start of the weeks. This can be understood if we go through and collect data regarding the growth that has happened over the period of time.