On Monday, all those believing in oneness of Allah, (Muslims) are granted pardon and forgiven by Allah, save those two persons, who have enmity and grudge against each other. So, on Mondays, keep the hearts and soul clean and purified, having enmity and grudge against none. It is good keeping a fast on Mondays as it is a sacred and blessed day; and also for the reason that it is “the day on which deeds and acts” are presented before Almighty Allah.
(Saheeh Muslim)