Encode the days of week as numeric variable
The answer is as follows:
If you are using a Linear Regression Model - then it will assume that Sunday (6) is bigger (or carries more weight) than all other days in the week. If that assumption makes sense for your problem domain - you are fine.
If you are using a model such as Random Forests - then it doesn't really matter (meaning you are good). Random Forests are typically scale invariant - They do not assume higher weightage for large values - They will figure out how best to split up the numeric values (0 - 6) to meet your objective.