Mr Saminejad is under arrest for the second time since November for writing about the arrest of other Iranian bloggers.
CPB is campaigning for the release of Mr Saminejad, one of 20 bloggers imprisoned by the Iranian authorities in the past year.
"Devote Thursday to an expression of sympathy for an innocent who has been unjustly detained," said a statement on the CPB site.
"Observe a fast on your blog for Mojtaba, linking to this page. Those among us who are healthy and so inclined might consider an actual day-long fast. When asked why, tell your family, friends, neighbours and co-workers about Mojtaba."
CPB has published full contact details for Iranian representatives in the US and government ministers in Iran and is encouraging bloggers to write in support of Mr Saminebad's release.
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