As you set up a Metric, you might notice that a number of the frequency options report on a certain day of the week. Take a look at the example below. The reporting day is on Thursdays:

The reason Thursday is the reporting day is because the start date of the Metric (10/14/21) falls on a Thursday. To change this, change the start date of the Metric (the first date in the Date Span selection) so that it falls on the desired day of the week.