In 2018 Burning Man 2018 took place at the end of August and beginning of September. Specifically, it began on August 26th and ended on September 3, 2018. Of course, the founders and supporters of Burning Man hope for people to come at the beginning of the event.
However, there are many events that take place within this main event. Participation is highly encouraged and expected. If you are creating art to be included in one of the parts or camps, then there are time restrictions. However, people can purchase tickets for any events and then they must be on time to participate. So, there are really no time restrictions for observers, but it is encouraged to be on time and participate wholeheartedly.