Answer

According to the widely accepted Jain traditional account, Bhagwan Mahavir attained Keval Gyan on the 10th day of the bright half of Vaishakh (Vaishakh Shukla Dashami), at Jrimbhikagrama on the bank of the Rijubalika river. His first discourse is traditionally said to have been given 66 days later, at Vipulachal near Rajgriha, on the first day of the dark half of Shravan (Shravan Krishna Pratipada / Ekam). ( jainworld.com)

Explanation

Jain tradition explains that Bhagwan Mahavir remained silent after Keval Gyan until he found the right first disciple, Indrabhuti Gautam, who became his first Ganadhara. That is why the first sermon was not immediately given on the same day. ( jainworld.jainworld.com)

Note

Some Jain references mention the first message as occurring on Vaishakh Shukla 11, but the more commonly repeated traditional account is the 66-day interval before the first discourse. ( jainworld.jainworld.com)

Takeaway

So, in short: Keval Gyan on Vaishakh Shukla 10, and first discourse after 66 days, at Vipulachal. ( jainworld.com)