The day of the week when Bitcoin is Lowest varies depending on a number of factors. Generally, however, it is Tuesday or Wednesday. The reason for this is that Monday is often when people buy Bitcoin after the weekend, and the price begins to drop on Tuesday or Wednesday as people sell Bitcoin for profit.
Fridays are also often when people buy Bitcoin in anticipation of the weekend, so the price may be higher then as well. Saturdays and Sundays are generally when the price is Lowest, as there is less trading activity during these days.
In conclusion, then, the day of the week when Bitcoin is Lowest varies depending on a number of factors, but is generally Tuesday or Wednesday.