Executive producer Lee Mendelson told USA Today this week that an ad executive at McCann-Erickson, whose client Coca-Cola (KO) was sponsoring the special, was unimpressed after looking at rough sketches and some animation tests that didn’t yet feature the now-iconic Vince Guaraldi jazz soundtrack. “This isn’t very good,” Mendelson said the exec told him before suggesting that Coke might cancel the show. But Mendelson assured him the project would come together and entreated the ad man to have faith in Schulz and his characters.

But even with the sponsor’s support, Mendelson said he and Schulz faced a tight deadline—just five months—to pull together what would become a holiday classic, which included the Peanuts gang putting on a play about the real meaning of Christmas, along with an under-appreciated Christmas tree that finds a champion in Charlie Brown.

Mendelson told the Los Angeles Times he was unsure how the public would receive A Charlie Brown Christmas, worrying that the cartoon might be “a little slow” and that it might flop without a laugh track—a feature nearly every comedy had in those days, but which Schulz reportedly hated. CBS (CBS), which originally carried the special, apparently wasn’t enthusiastic either. After previewing the show before it aired, according to Mendelson, the network said, “Well, you gave it a good try.”

Once the special aired on Dec. 9, 1965, though, it saw immediate success, receiving a rating share of nearly 50, which meant that half of the country tuned in to watch. “It became part of everybody’s Christmas holidays,” Mendelson told the Times.

A Charlie Brown Christmas marks its 50th anniversary with a two-hour block of programming on ABC (DIS) Monday night. Here are some more facts about the show:

• The special’s surprising success inspired CBS to order a slate of additional Peanuts specials, including A Charlie Brown Thanksgiving and the Halloween-themed It’s the Great Pumpkin, Charlie Brown.
• The special aired on CBS every year from 1965 through 2000, at which point ABC acquired the rights to the Peanuts specials.
• The Christmas special continues to generate strong ratings for the network, pulling in more than 6 million viewers last year.
• As we mentioned above, Coca-Cola sponsored the special’s creation, and the show originally contained multiple plugs for the beverage company that were eventually removed for later airings.
• While CBS execs may not have been sold on Guaraldi’s jazz accompaniment, the soundtrack—which featured a California children’s choir singing on now-classic tunes such as “Christmastime is Here”—has reportedly sold more than 3 million copies, according to Nielsen SoundScan, while finding its way into the Library of Congress and the Grammy Hall of Fame.
• This year, the U.S. Postal Service issued holiday-themed stamps featuring scenes from A Charlie Brown Christmas in honor of the special’s 50th anniversary.
• While it may not be holiday-themed, the recently-released The Peanuts Movie has also proven to be popular with audiences, as the animated feature has pulled in more than $125 million in worldwide ticket sales in less than a month in theaters.