Upcoming PR update dates (all are approximate and based on previous updates)
Year 2012 > Next Google PageRank update Date 2012
Year 2013> Next Google PageRank Update 2013 | Next PR update expected dates
NOTE: all the dates mentioned in those posts are based on previous update data, and in the world no one (except google) can exactly say the exact PR update dates.
Though the above dates are pity much accurate.