Bhagavan Sri Krishna’s incarnation was in the Chandra Vamsam (Lunar dynasty). In the Chandra Vamsam, during the period of Chandra Udayam (moonrise), Chandra Dinam (Monday), Bhagavan Sri Krishna’s Avatara took place. He is known by the names Yaduchandran, Yadukula Chandran, etc. That is why we give importance to Chandran (the moon) on that day.