Nepalese women undertake these rituals in order to achieve what is desired by women everywhere - a happy and productive marriage, good fortune, a long life for their husbands, and the purification of their own bodies and souls.  Traditionally, the ritual of Teej is obligatory for all Hindu married women and girls who have reached puberty.  Exception is made for the ones who are ill, or physically unfit.  In such circumstances, a priest performs the rites.  They feast, fast, and get high, they dance in their red wedding saris, and finally they imbibe their divine husbands' foot-bath.
Previously, this ritual was observed by upper caste women primarily.  Nowadays, everybody wants to get in the act.  Here we have the Newari wife of our Maoist Prime Minister (a brahmin) taking a turn at the dances of Teej, which could be seen as a paean to patriarchy -

The first day of Teej is called the "Dar Khane Din".  On this day the women, both married and unmarried, assemble at one place, in there finest attires, and start dancing and singing devotional songs.  Amidst all this, the grand feast takes place.  The jollity often goes on till midnight, after which the 24-hour fast commences.  Some women go without a morsel of food or a drop of water, while others take liquid and fruit.
Gaily dressed women can be seen dancing and singing on the street leading to Shiva temples. But the main activities take place around the Pashupatinath temple where women circumambulate the Lingam, the phallic symbol of the Lord, offering flowers, sweets and coins.

The third day of the festival is Rishi Panchami.  After the completion of the previous day's puja, women pay homage to various deities and bathe with red mud found on the roots of the sacred Datiwan bush, along with its leaves.  This act of purification is the final ritual of Teej, after which women are considered absolved from all sins.  The recent years have witnessed alteration in the rituals, especially concerning their severity, but its essence remains. No matter how agonizing the fast may be Nepalese women have and will always continue to have faith in the austerities of Teej.

 Legend tells us that Brahma prayed to Ganesha for success in His work of creation.  When the moon Goddess Chandrama, rebuked the Creator for praying to a mere godling, Ganesha cursed her that anyone who looked upon her would become a thief.  It took the intercession of all the other divinities to placate the offended Ganesha.  Now we can look upon the moon and bask in her gentle rays, except on this night which is only auspicious for robbers.  Tonight people closet themselves in their houses, shut the windows and doors and even stuff the cracks so that the moonlight cannot penetrate inside with disastrous consequences.  Hopefully, it will be overcast.  The only people out are thieves and robbers (and the police, who after all are part-time extortionists...).  A crook who doesn't get some loot on this night is most unfortunate and will probably soon get busted.  So beware.