Here every day can be a good day to buy or a good day to sell....Not really a particular day of the week.
Who is investing and how big is the investment generally determines dip and rise in the value. If allot of people start buying bitcoin whose price is increasing thinking that it will further increase, it increases for a spike and stabalises, and when slight decrease is seen, people who are panic sellers tend to start selling and due to this there is sudden dip in tha value which and then it doesnt dip further below.
This anticipation brings all the change and dips and rises.
And in case of bitcoins i dont think any special particular day is decided for rise or fall.