Snoopy began befriending birds in the early 1960s, when they started using his doghouse for various purposes: a rest stop during migrations, a nesting site, or a place to play cards. None of these birds were ever given names, or even used speech balloons; they simply looked at Snoopy and he understood them. What set Woodstock apart from all these earlier birds was the fact that he attached himself to Snoopy and assumed the role of Snoopy’s sidekick and assistant. There had been no recurring relationships between Snoopy and the earlier birds who visited the yard of the Brown family, and Snoopy was as often as not more hostile than friendly toward those birds. But, in the April 4, 1967, Peanuts daily comic strip, a single bird flew in after a long flight while Snoopy was lying on top of his dog house. He chose Snoopy’s nose as a good place to rest, and Snoopy uncharacteristically accepted this intrusion. Over the next two days, Charles Schulz began to establish character traits for Snoopy’s new friend by revealing that he could talk (more accurately that he could complain, in the form of repetitive sounds in word form —”gripe, gripe, gripe, gripe”, “complain, complain, …”), that, unlike normal birds, he didn’t like to fly south every winter, and that his flying skills were not quite up to snuff. By the end of this four-strip sequence, Snoopy, in character as the World War I Flying Ace, learns that the bird is his new mechanic — Woodstock’s first supporting role. After this introduction, the unnamed Woodstock is seen with Snoopy on occasion, and other birds continue to appear as they had for years. But Woodstock is singled out as the bird who befriended Snoopy, in part by continuing references to him as the Flying Ace’s mechanic (July 12, 1967; June 12–14, 1968). Finally, on June 14, 1968, fourteen months after his first landing on Snoopy and after a second appearance as a supporting character for Snoopy (his wrist wrestling partner on April 25, 1968), the most important aspect of Woodstock’s relationship with Snoopy is made clear — Snoopy first refers to this bird as his buddy. That identification was more than enough for readers to know, if they hadn’t already figured it out, that this little bird, name or no name, had assumed the role of a regular character in the Peanuts cast.
Schulz did not give him a name until June 22, 1970. Schulz acknowledged in several print and TV interviews in the mid-1970s that he took Woodstock’s name from the rock festival. (The festival’s logo showed a bird perched on a guitar.) [Excerpt adapted from Wikipedia, the free encyclopedia.]