There is no definitive answer to this question, as many factors can affect which day is best for you to go to the laundromat.
However, some tips that may help you decide include considering the days when the laundromat is likely to be less busy and choosing a time when you can quickly get there and back.
Additionally, checking the laundromat's hours of operation in advance may be helpful to make sure it will be open when you need to go.