First you want to create a duplicate (in my case monday - friday (2) that does NOT have a relation to you source table. You want this duplicate table to only show days. This duplicate table needs a relationship with the slicer you are going to use to filter for days
In your source table (monday - friday) you want to create the following measure:
Measure = IF(HASONEVALUE('monday -friday (2)'[Day]);
IF(SELECTEDVALUE('monday -friday (2)'[Day]) = MAX('monday -friday'[Day]); 1; 0);
IF(ISFILTERED('monday -friday (2)'[Day]) && COUNTROWS(FILTER('monday -friday (2)'; 'monday -friday (2)'[Day] = MAX('monday -friday'[Day]))); 1; 0))
Now you want to create conditional formatting. First create your table with days and values (for the example i did not sort the table. You want to sort your table so it looks better):
Then right click on values and select conditional formatting. Select based on rules.
and the measure you just created in your source table.
for the first option you select "is bigger than or equal to" and give it a value 1.
for the second option you select "is smaller than"and give it a value 1000.
Choose the color you want your highlights to be.
If you now select a day in your filter the value for that day will show the background color you selected! In your case you want to select all days you want to have highlighted.
Hope this helps!