April 15 is the popular CCP CC General Secretary Hu Yaobang's death anniversary. This year is his 30th death anniversary and the Chinese leadership has retained heightened security precautions implemented for the 'Big Two' sessions in the first fortnight of March.  

(Comment:  Other sensitive anniversaries this year are the 100th anniversary of the May 4 Movement, the 30th anniversary of June 4, and the 70th anniversary of the founding of the PRC on October 1.)