Article: Les Demoiselles d'Avignon
Section: Rivalry with Matisse

Question: When did he join the Kahnweiler art gallery?

The Salon d'Automne of 1905 brought notoriety and attention to the works of Henri Matisse and the Les Fauves group.
The latter gained their name after critic Louis Vauxcelles described their work with the phrase "Donatello au milieu des fauves!"
("Donatello among the wild beasts"), contrasting the paintings with a Renaissance-type sculpture that shared the room with them.
Henri Rousseau (1844–1910), an artist that Picasso knew and admired and who was not a Fauve, had his large jungle scene "The Hungry Lion Throws Itself on the Antelope" also hanging near the works by Matisse and which may have had an influence on the particular sarcastic term used in the press.
Vauxcelles' comment was printed on 17 October 1905 in the daily newspaper "Gil Blas", and passed into popular usage.
Although the pictures were widely derided—"A pot of paint has been flung in the face of the public", declared the critic Camille Mauclair (1872–1945)—they also attracted some favorable attention.
The painting that was singled out for the most attacks was Matisse's "Woman with a Hat"; the purchase of this work by Gertrude and Leo Stein had a very positive effect on Matisse, who was suffering demoralization from the bad reception of his work.
Matisse's notoriety and preeminence as the leader of the new movement in modern painting continued to build throughout 1906 and 1907, and Matisse attracted a following of artists including Georges Braque (1880–1963), André Derain (1880–1954), Maurice de Vlaminck (1876–1958).
Picasso's work had passed through his Blue period and his Rose period and while he had a considerable following his reputation was tame in comparison to his rival Matisse.
To make matters worse Matisse shocked the French public again at the 1907 Société des Artistes Indépendants when he exhibited his painting "Blue Nude (Souvenir de Biskra)," completed in early 1907.
The "Blue Nude" was one of the paintings that would later create an international sensation at the Armory Show of 1913 in New York City.
By the spring of 1907 when he began to paint "Les Demoiselles d'Avignon", until its completion later in the year, Picasso was vying with Matisse for the preeminent position of being the perceived new leader of Modern painting.
Upon its completion the shock and the impact of the painting propelled Picasso into the center of controversy and all but knocked Matisse and Fauvism off the map, virtually ending the movement by the following year.
In 1907 Picasso joined the art gallery that had recently been opened in Paris by Daniel-Henry Kahnweiler (1884–1979).
Kahnweiler was a German art historian, art collector who became one of the premier French art dealers of the 20th century.
He became prominent in Paris beginning in 1907 for being among the first champions of Pablo Picasso, and especially his painting "Les Demoiselles d'Avignon."
Before 1910 Picasso was already being recognized as one of the important leaders of Modern art alongside Henri Matisse who had been the undisputed leader of Fauvism and who was more than ten years older than he was and his contemporaries the Fauvist André Derain and the former Fauvist and fellow Cubist, Georges Braque.
In his 1992 essay "Reflections on Matisse", the art critic Hilton Kramer wrote, 
After the impact of "Les Demoiselles d'Avignon", however, Matisse was never again mistaken for an avant-garde incendiary.
With the bizarre painting that appalled and electrified the cognoscenti, which understood the "Les Demoiselles" was at once a response to Matisse's "Le bonheur de vivre" (1905–1906) and an assault upon the tradition from which it derived, Picasso effectively appropriated the role of avant-garde "wild beast"—a role that, as far as public opinion was concerned, he was never to relinquish.
Kramer goes on to say, 
Whereas Matisse had drawn upon a long tradition of European painting—from Giorgione, Poussin, and Watteau to Ingres, Cézanne, and Gauguin—to create a modern version of a pastoral paradise in "Le bonheur de vivre", Picasso had turned to an alien tradition of primitive art to create in "Les Demoiselles" a netherworld of strange gods and violent emotions.
As between the mythological nymphs of "Le bonheur de vivre" and the grotesque effigies of "Les Demoiselles," there was no question as to which was the more shocking or more intended to be shocking.
Picasso had unleashed a vein of feeling that was to have immense consequences for the art and culture of the modern era while Matisse's ambition came to seem, as he said in his "Notes of a Painter," more "limited"—limited that is, to the realm of aesthetic pleasure.
There was thus opened up, in the very first decade of the century and in the work of its two greatest artists, the chasm that has continued to divide the art of the modern era down to our own time.
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