On the day your session will take place. We'd appreciate getting your finished presentation 2 days before your talk though.
Only your short session description will be published online. We’d suggest making this short description informative and well formulated when submitting, in order to get us and the conference participants interested in your session. Also, make it short but sweet (hardly anyone reads more than half a page, so your session's description is limited to 2,000 caracters).