They call it "Knott's Merry Farm" during the Christmas season

, and "Knott's Scary Farm" during the Halloween season.
Yes, it is Knott's Merry Farm during the Holiday season, but don't confuse it with Knott's Scary Farm. Knott's Scary Farm is an entirely separate event from visiting the park on just any Halloween day. The park closes and you have to have a separate ticket. It is wildly popular and a tradition for many HS age kids from all over Southern California.
Knott's Merry Farm is the regular theme park decked out for Christmas. The parade is called "The Spirit of the Season" and it is very small compared to Disney parades but they do have floats with Snoopy and the gang and if you're there before Christmas, the final float includes Santa. After Christmas, Snoopy sits on the final float.
Another popular Christmas time tradition at Knott's is "Snoopy's Christmas on ice". This show tends to draw a large audience every time and is really very fun and yes ... it takes place on a stage that is actually a small ice rink. There are rumors this show might change for next year, but I haven't heard anything firm.
The log ride is converted into a Christmas theme and Camp Snoopy tends to be very well done for the holidays. I don't recall any other rides being redone for the holiday theme.
If you enjoy live theater, don't miss the bird-cage theater shows. There are two. One is the "Magi" show and the other is "A Christmas Carol". These are shorter versions of what you might see on a larger stage, but the actors are typically excellent and these two shows are a surprise gem.
Wondering the park are "The Calico Carolers". These are from the same company that supply the Disney carolers and if you go often enough you may even recognize some of the same people, but Knott's doesn't use the bells.