There was a study done that showed that stations (primarily Mainstream A/C) that go Christmas generally get a 10 percent spike in the all important fall rating period. It has proven to make a difference.
Also..the rating period for the Fall ends on December 12th...a full 13 days before the actual Christmas Holiday. There is interest in Christmas music once Black Friday hits.
And, depending on the market, a station may need to go earlier.