By tradition, when does the presidential campaign begin?

The presidential campaign formally opens when a candidate is officially nominated at the National convention. thereafter, the candidate can call out press conferences, conventions to call out his platforms for everyone to know his intentions about the country. The republic allows all legible voters to vote their own cadidate of choice. 

Is there a passage that comes with this question or should i know this right off the bat, because if it is, i probably have short term memory loss or college is too stressful