Shalom Sam:

The difference in the Shavuot observance really begins with one's interpretation of the First Fruit observance.

The orthodox Jews observe First Fruits right after Pesach, Nisan 16, hence Shavuot for them this year is May 25.  This manner of observance links them
to the lunar calendar so Shavuot always "moves" in the week from one year to the next.

When one starts from the first day of the week (Sunday) after Pesach, as written in The World, then Shavuot comes 50 days later, and again on the
first day of the week.  This year April 11 was that first day of the week following Pesach, therefore May 30 this year will be Shavuot for us.

The Christian Chuch's Easter came on First Fruits this year so they will observe their Pentecost on May 30 also.

Therefore your observance of Shavuot depends on when you observe First Fruits!

Dianne