Here are some of the many Events that are happening in Sid Story.
Season Events are the Exceptionals Season held every two weeks. There are always 2 Sidians you can meet and sometimes a 3rd one.(hidden drop or low chance) 1 Exceptional can be met in your FEED by sending Sidians for Exploration at the Gate. 1 Exceptional can be met by taking classes at the School.
The Special Lessons are limited classes that appear in the School menu. In these lessons, the teacher can be a limited time Sidian that you can acquire if you complete the 3 Lesson Grades.
Mileage Events usually have the same structure, where you get a reward based on points you get after every battle, according to a point milestone.