Mister Magoo's Christmas Carol debuted two years' before, but Rankin-Bass's Rudolph the Red-Nosed Reindeer is the earliest Christmas special I remember. "The most famous reindeer of all's" story premiered on December 6, 1964 as an entry in General Electric's Fantasy Hour, a series of color specials that aired on Sunday afternoon. Someone has posted the original opening and commercials (featuring Santa's elves) from that first appearance to YouTube:
The closing includes a couple of nice shots of the voice cast. Jerry Beck, at his invaluable blog Cartoon Brew, has identified them. The first photo shows, from left to right, Janice Orenstein (Clarice), Paul Kliegman (Donner and the reindeer coach) and Paul Soles (Hermey). From left to right in the second photo: Soles, Billie Mae Richards (Rudolph), Carl Banas (Head Elf and Spotted Elephant) and Alfie Scopp (Charlie-in-the-Box).
I'd forgotten that this wasn't a prime-time entry, but its popularity ensured that it would become one for Christmas Future. It received a lot of press back in 1964, and more than one reviewer opined that we would see Rudolph annually... as indeed we have.
To all my readers: May you have a Merry Christmas and a Happy and Prosperous 2011. And to quote from the Christmas special I didn't see first, May God Bless Us, Every One.