Valmiki (during Lord Rama's time) is celebrated as the poet harbinger in Sanskrit literature. He is the author of the epic Ramayana, based on the attribution in the text of the epic itself. He is revered as the Adi Kavi, which means First Poet, for he discovered the first śloka i.e. first verse, which set the base and defined the form to Sanskrit poetry.
Read more about Valmiki: Early Life, How He Became Valmiki, Writer of The Ramayana, The First śloka, Honorary Holiday