I don't believe in such things, come man maybe it was just a coincidence that the marks had happened always on Monday. For me the best day to buy bitcoin is when you see it on sale and you have the money to buy at your fullest.
Its best to watch the current status of Bitcoin on the chart. Then you can understand exactly whether to buy it or not. This is logical. Due to the volatility, I cannot consider any of the days of the week the most suitable, because the price is changeable