The indicator shows the current day of the week as a histogram. The histogram rule matches the numbering of days in the MqlDateTime structure:

struct MqlDateTime     {      int year;           // year      int mon;            // month     int day;            // day     int hour;           // hour     int min;            // minutes     int sec;            // seconds     int day_of_week;    // day of the week (0-Sunday, 1-Monday, ... ,6-Saturday)     int day_of_year;    // number of a day in the year (1st of January has number 0)    };  

The "Days of the week" indicator can be applied on the D1 timeframe together with "Volumes" indicator for a visual search of dependencies between days of the week and volumes:

Fig. 1. An example of a combination of "Days of the week" and "Volumes"