The Mythic events are separate from the Legendary events.
The Legendary Events are ~7 day long events that allow players to earn character shards for the Legendary character. Mythic Events are usually 2 day long events that are only available to players who already have the character at 7 stars.
Usually, Mythic Events appear in the middle of the corresponding Legendary event (and almost always on the Saturday/Sunday in the middle of the Legendary). However, it appears that the Devs turned on every current available Mythic Event for the holidays, and staggered them on a different schedule.
Hopefully the January Event Calendar will be posted soon. It's been almost 3 months since both R2 & BB8's legendary events have returned, so they are both likely to return fairly soon!
