weekday-from-dateTime() function extracts the day of the week (as a number) from a Date and time attribute so it can be used to compare to a value. Values range from 1 to 7 (1 = Sunday, 7 = Saturday).
This query returns all the logs where the day of the week in
DateAttribute is 6 (Friday):
//Logging.Log[weekday-from-dateTime(DateAttribute) = 6]