Hello Momo, to attend: means to be present without necessarily being involved or having share in the activity. For example not all attendants have to be professors of mathematics. Usually festivals,meetings, classes, concerts and weddings are attended. You could be taking part in the occasion or just invited whether you are playing any rule or not. to participate: being present but involves necessarily taking a positive part in the event like contributing to the expenses for example. "e.g to participate in sport events". In your example attending the annual Geometry Festival could be done by a variety of people including professionals , professors,, sponsors and mere amateurs, etc. So here you will be using "attending"!

attending -the person will go, physically appear participating - to take part in an event or activity

This sentence is unclear. Either word would work fine. If you use "attending", it will mean that the person will go and observe the Festival. If you use "participating", it will mean that the person will go and present an exhibit. The sentence does not make that clear, so either word works.

It is actually participating since it includes the word "enroll" at the end of the sentence meaning that the person who wishes to go must display something while attending the event.