First the developer Mojang altered Minecon to become Minecraft Festival, which was supposed to happen in September of this year. Now the event has been postponed until next year because of concerns regarding coronavirus (COVID-19).
According to a statement, "September is still many months away, and we are not making any predictions about how long it will take to put the outbreak behind us. This decision is rather a result of the extensive preparations required to organize a mammoth event like this."
By postponing the event the organizers "can make sure that next year's event will be the amazing one that our community expects and deserves".
Minecraft Live, however, "will happen, as planned". Details about this live stream event will be shared at some point in the future.