Every day is a holiday - just check Twitter for proof.
For example, at the time of this writing one "holiday," #NationalBobbleheadDay has been trending for several hours. Although this one in particular may not be useful for the majority of Internet marketers (unless they actually sell bobblehead dolls or happen to have a few around the office that can be used creatively in a social post), there are many other holidays (outside of those in November and December) that can be leveraged in social media campaigns.