How the date is fixed
Hartalika Teej falls on the third day of the bright half of the lunar month Bhadrapada (Shukla Tritiya). The date is the day on which that tithi is running at sunrise (the usual rule, called udaya tithi).
About
Hartalika Teej falls on the third tithi of the bright half of Bhadrapada, in August or September, the day before Ganesh Chaturthi. The vow recalls Parvati's austerity to win Shiva, and the name refers to her friends (ali) who carried her off (harit) to the forest. The tithi at sunrise is taken; when Tritiya is joined with Hasta nakshatra the day is considered complete. Karnataka keeps the same day as Gowri Habba, and the Haryali and Kajari Teej of Shravana are separate observances.
How it is observed
A strict fast, often without water, worship of clay images of Shiva and Parvati, and a night of song.