... but could you please tell us HOW to use this? ...
There are two ways to do it. For example, you can do it one time and it will cover every birth event in your database, or you can do it for each birth event in your database, one at a time. In normal circumstances, you would want to do it one time and cover every birth event in your database. You would do it for each birth event in your database, one at a time, only if you wanted to do it for some birth events but not for others.
So here is what you would do for the birth event. Go to the Fact Type List from the menu bar via Lists->Fact Type List... Double click Birth from the list (or single click Birth and then single click the Edit button). Double click Principal in the list of roles, or single click Principal and single click the Edit button.
At this point, the default sentence template for the Birth event will be displayed, and you can edit it to be anything you wish. The default is as follows.
[person] was born< [Date]>< [PlaceDetails]>< [Place]>.
Change it to say the following.
[person] was born< [Date:DayOfWeek]>< [PlaceDetails]>< [Place]>.
Click OK, OK, and Close, and you will be done with the birth fact. Do the same thing with any of the other fact types for which you wish the day of week to appear.
Suppose instead that you only wish the day of week to be displayed for the birth event for John Doe. Go to John Doe and double click him. You will be in his Edit Person screen. Double click his birth fact. On the far right hand of the screen at the bottom of the data for John you will see text that says Customize sentence. Single click on that text and you will see the sentence template that's just for John. Make the change there, and it will be changed just for John. You can do the same thing on a case by case for anybody else in your database, but normally you will want to use the first method because it applies to everybody in your database all at one go.