Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) said:
The hadeeth of ‘Abd-Allaah ibn ‘Amr (may Allaah be pleased with him) indicates that if fasting on a Friday or Saturday coincides with a day one usually fasts, without intending to single that day out for fasting, then there is nothing wrong with it, because if he fasts alternate days, then that will coincide with some Fridays and Saturdays. Thus it becomes clear that fasting on these days is not haraam. Otherwise the Prophet (peace and blessings of Allaah be upon him) would have said: Fast one day and not the next, so long as that (fasting) does not coincide with a Friday or Saturday.
Al-Sharh al-Mumti’, 6/476.