Michael Colunga (10 May 2011)
"Since the vernal equinox was on March 20 at 23:21 UTC, the correct day for Nisan 1, 5771 was April 5, 2011, having begun at twilight, April 4, 2011."

 
Hello, John and Doves,
 
Since the vernal equinox was on March 20 at 23:21 UTC, the correct day for Nisan 1, 5771 was April 5, 2011, having begun at twilight, April 4, 2011.  http://earthsky.org/astronomy-essentials/everything-you-need-to-know-vernal-or-spring-equinox
 
Thus, the correct day for Pentecost is Sivan 6, 5771/June 8, 2011.  http://www.aish.com/h/  You can download the calendar at the bottom of the page.  Note that this is a strictly Jewish site, so they have no ax to grind.
 
Baruch HaShem Adonai,
Mike C.