The Feast of Weeks - Pentecost, was to be celebrated 50 days after the Feast of First Fruits. Both were always celebrated on a Sunday. Contemporary Jewish practice yields to traditions other than the Torah. Shavuot is observed on the 6th of Sivan (May 26, 2004) in current practice, which is in error.
 
This is explained in detail in the Karaite Korner website at http://www.karaite-korner.org/shavuot.shtml.