No-one really knows when the first festival took place. Mankind has been partying and enjoying himself every since the dawn of time!
We know that ancient people worshipped deities associated with the sun and moon, and that these, then, would be celebrated. Since food meant survival, however, then the harvest itself would be the greatest cause for celebration, and would include all levels of society.
We still give thanks at harvest time, of course, and this is one reason for a street party.
Another, in some countries, is to honour national pride, perhaps in the form of a president, monarch, or date of independence.
Gods and goddesses are the impetus for many celebrations still, however.
Street festivals have existed since before streets were created, therefore! It’s in our human nature to give thanks in times of bounty and for those who we believe ensure our survival.