Sri Sri Radha Krishna-chandra after flower petal abhisheka
Pushya Abhisheka is a festival where the Lord is bathed in soft and cooling petals of fragrant flowers. The Hari Bhakti Vilasa says that on this day one should rub ghee on the deity of Sri Krishna. Pushya means nourishing, and this act of devotion, with ghee, which is very nourishing is equivalent to the results of an ashvamedha sacrifice.
Srila Prabhupada once explained the festival in the following way: "Krishna was just a toy in the hands of the gopis, so one day the gopis decided to decorate Him. Pushya Abhisheka means a ceremony to decorate the deity profusely with flowers, ornaments and clothes. After that there should be lavish feasting and a procession through the streets, so that all the citizens should see how beautiful Krishna appears."
On the purnima of the month of pausha (December-January) (Srimad Bhagavatam, 2:10:4, 'posanam tad-anugrahah), the last day of the month of Pushyami nakshatra, one should bathe the deity with ghee.
The Skanda Purana 2.2 chapter 42 describes Sri Krishna Pushya abhisheka as one of the festivals observed for Lord Jagannatha in Puri. This chapter gives an elaborate description of the rituals for observing the festival.
In the 27th chapter text 99 of the same section of Skanda Purana the pushya nakshatra is also mentioned. There it is said that it was on a Thursday, ashtami tithi, during shukla-paksha of the month of Vaishakha in conjunction with the pushya-nakshatra that Lord Jagannatha was installed in the original temple in Puri.
Srila Prabhupada instructed devotees to celebrate festivals related to the pastimes of Sri Sri Radha-Krishna in a grand way. These pastimes are going on eternally in the spiritual world. By celebrating these festivals, we can engage ourselves directly in the service of the Lord.