The lack of traffic on the streets or of people on the square would indicate that the photo was taken very late at night. I am sure that as I write this, the enormous plaza is filling up with people. By tonight, the eve of Independence Day, the Zócalo will be jam-packed with thousands.
The large building with several courtyards in the lower left hand corner is the National Palace. At 11:00 tonight the President will appear on a balcony facing the Zócalo and ring the bell which was rung on September 16, 1810 to begin Mexico's fight for independence from Spain. The President will then give the traditional "Grito de Independencia"... the Cry of Independence.
Long live the heroes that gave us our fatherland and liberty!
(He then lists the heroes of the War of Independence.)
Long live Hidalgo!
Long live Morelos!
Long live Josefa Ortiz de Domínguez!
Long live Allende!
Long live Aldama and Matamoros!
Long live national independence!
Long live Mexico! Long live Mexico! Long live Mexico!