The first day of the week is called Somavar, and Soma means Moon. According to the Puranas, it is believed that when Saint Prajapati had cursed the moon to remove it, he went to Shiva for the liberation of the curse.
The day of the incident was Monday, Sati her sisters and some Rishi started chanting the mantra, of Maha Mrityanjya Mantra that, was given to them by Rishi Markandeya. Group chanted the mantras whole night and kept awake.
Delighted by the passion and devotion of everyone, Shiva rescued moon from the curse of Daksh, gave the moon a permanent place in his hair and thus got the name Chandrasekhar. Like this Monday became famous amongst Shiva disciples and was considered auspicious for worshiping.