When was the first period of cubism?

Picasso's Cubist Period consisted of Analytic Cubism, Synthetic Cubism, and sometimes Proto-Cubist works. Analytic Cubism was about capturing the 4th dimension, or depicting an object on all sides simultaneously. It was about seeing the world through a shattered mirror and attempting to put the pieces back together. Take a look at Portrait of Ambroise Vollard (1910).

Synthetic Cubism focused on emphasizing the flatness of a 2-D surface by incorporating letters, numbers, and real objects into works. Art of Synthetic Cubism sought to highlight the contrast between 2-D and 3-D. See Still Life With Chair Caning (1912) for an example. Picasso used letters and real rope in this work to bring out the contrast between the flat canvas and the real objects.

Proto-Cubist works exhibited gradual progressions towards more abstract Cubist works, which is why they could be considered as part of Picasso's Cubist Period. For example, his Les Demoiselles d'Avignon (1907) shows the progress towards abstraction as the viewer focuses his or her attention on each of the women in the painting.