Haydn's Symphony No. 1 may or may not have actually been his first foray into this new musical form, but it nonetheless was a pioneering work, containing elements of both the Baroque as well as the new Classical styles. Haydn was doing some tinkering here, and he would quickly develop a model that would become the symphony as it has been defined ever since.
It's where we begin our program this week, and if it's status as the first symphony is questionable, there is no argument that it is a transitional work of music.