Let's pick it apart a bit:
"Is March National Something Month?" Means "Is March a month officially consecrated to the celebration or memory of Something?"
The Truth Movement is some sort of 9/11 conspiracy theory organization, presumably.
"To beat a dead horse" is an common idiom for wasting time on a futile and already failed effort. Eg. in this case the mission to show the "sheeple" that 9/11 was an inside job.
So, the author is suggesting that March was a month during which the Truth Movement was particularly vigorous in advancing their ideas in the United States.
The source text is actually formed from a fusion of two common idioms: that of beating a dead horse and that of a National Something-or-Other Month.
The practice of observing or commemorating months is still very much alive and well:
(Note that International Something Day/Month/Year is also common. The United Nations is still busy issuing observances)
A look at Google Ngrams is sufficient to show that our poor dead horse continues to suffer.