According to Padma Purana, Srstikhanda, Chapter 17, a demon named Vajranash was creating havoc on earth and killing people.
Lord Brahma had to step in to stop the demon from his acts.
While fighting the demon, three petals from the lotus in his hands fell on Earth. The place was none other than what we know today as Pushkar Rajasthan.
The fallen petals immediately transformed into three lakes, and they were dedicated to the trinity.
While Jyestha is Pushkar Sarovar, dedicated to Lord Brahma, Mad
