On May 20, 2011 at 18:50 MSK (14:50 UTC), the K-84 Ekaterinburg submarine of the Project 667BDRM class successfully launched a R-29RM Sineva missile. The launch was performed from a submerged submarine deployed in Barents Sea. The Navy reported that all warheads reached their intended targets at the Kura test site in Kamchatka.
Previous Sineva launch took place less than a month ago, in April 2011, from the same submarine.
A report about a Sineva launch appeared in online publication Navy.ru on May 18, 2011. It appears that that report described the launch that actually took place on May 20th. The early release of the information may suggest that the launch was originally scheduled for May 17th, but then was postponed.
UPDATE: Makeyev Design Bureau reported that it was a launch of a Liner SLBM.