The correct Pentecost (Shavuot) according to Scripture falls on the 29th day of the 4th Hebrew month, which in 2011 falls on July 30/31st.
In Bible times, there was no Gregorian calendar. There were three types of days: work days, Sabbaths and New Moon days. All Sabbaths fell on the 8th, 15th, 22nd and 29th day of the Hebrew month.
Philo kept the Lunar Sabbath. Check here: http://lunarsabbath.us/id3.html and here: http://lunarsabbath.info/id3.html