Shama-il Tirmidhi Chapter 042, Hadith Number 009 (289).

'Aayeshah Radiyallahu 'Anha reports: "Rasulullah Sallallahu 'Alayhi Wasallam fasted three days of every month. In some months he fasted on Saturdays, Sundays and Mondays, and in some months he fasted on Tuesdays, Wednesdays and Thursdays".

Commentary
So that in this manner all the days of the week are covered. The days of Friday were intentionally omitted as stated in some ahaadith that this day had been proclaimed as an 'Eid. Other important matters were fulfilled on this day. Or Friday has not been mentioned in this narration and may have been mentioned in other narrations.