There are several options:
1) If you have "Use date" checked and "Week days" unchecked - the task will start only once on that day (unless you have "Repeat every ... minutes" feature enabled).
2) If both "Use date" and "Week days" are checked, then task will start every day after specified date. Of course, it will start only on week days which are checked - Sunday, Monday, etc.
In this case, you should consider the date as a "Start date".