Devasnana Purnima is an inseparable part of the famous Rath Yatra Puri. The day is believed to be the birthday of the Lord Jagannatha. This day, the full-moon day of the month of Jyestha, the deities are bathed by 108 pitchers of water of the well situated near the Sitala temple in the confines of the Jagannatha temple. Having been bathed, the deities are dressed up in the Gajanana or Ganesha Vesha.
Also Read: Full details on Jagannath Nabakalebar here