The billboard will be up for a month and is meant to counter the "Keep Christ in Christmas" display by the Knights of Columbus that the borough seems to think is appreciated by persons of non-Christian religions and those who follow no creed at all.
MORE: http://www.patheos.com/blogs/friendlyatheist/2013/12/13/keep-saturn-in-saturnalia-billboard-counters-christian-sign-in-new-jersey-town/
See also...
Last Night, the 'Keep Saturn in Saturnalia' Billboard Was Nearly Set on Fire
http://www.patheos.com/blogs/friendlyatheist/2013/12/18/last-night-the-keep-saturn-in-saturnalia-billboard-was-nearly-set-on-fire/
Police seek pair who tried to set fire to New Jersey 'Saturnalia' atheism billboard
http://www.rawstory.com/rs/2013/12/18/police-seek-pair-who-tried-to-set-fire-to-new-jersey-saturnalia-atheism-billboard/