Daihasei Festival Arc

The refers to two major story arcs that occur at the same time in both the Toaru Majutsu no Index and Toaru Kagaku no Railgun timelines focusing on the events of the Daihaseisai.

Below are the links for the arcs of both series:
 * Daihasei Festival Arc (Index) - Focuses on the secret events during the Daihaseisai, wherein Kamijou Touma — and the company of Necessarius magicians — deal with magicians of the Roman Catholic Church trying to take over Academy City during the festival. Mostly concentrates on the Magic side, with minute science scenes scattered around the entire arc.
 * Daihasei Festival Arc (Railgun) - Focuses on Misaka Mikoto's scuffles with MEMBER and Shokuhou Misaki as the both of them try to have their way, one of Mikoto's sisters, for their own schemes at the height of the events of the festivals.