The National Labor Relations Board (NLRB) has postponed for a second time the date by which employers must post a workplace notice informing employees of their right to join a union.

The NLRB announced last week that it has changed the effective date of its employee rights notice-posting rule from January 31 to April 30, 2012. The new date was established at the request of a federal court in Washington, D.C. that is hearing a legal challenge to the posting requirement from the National Association of Manufacturers (NAM).

“The Board’s ruling states that it has determined that postponing the effective date of the rule would facilitate the resolution of the legal challenges that have been filed with respect to the rule,” the board said in a press release posted on its Web site.

The NLRB previously postponed implementation of the posting rule from November 14 to the January 31 date to allow for what it called “enhanced education and outreach to employers, particularly those who operate small and medium sized businesses.”

The union notification rule will require private-sector employers whose workplaces fall under the National Labor Relations Act to post an employee rights notice where other workplace notices are typically posted, including Internet or Intranet sites.

The notice, which is similar to one required by the U.S. Department of Labor for federal contractors, states that employees have the right to act together to improve wages and working conditions, to form, join and assist a union, to bargain collectively with their employer, and to refrain from any of these activities. It provides examples of unlawful employer and union conduct and instructs employees how to contact the NLRB with questions or complaints.

Associated Industries of Massachusetts has opposed the notification rule. The association will send out more information on the poster status as it develops.