I found the answer from Catholic News Service:

*"At the Council of Nicaea in 325, all the Churches agreed that Easter,
the Christian Passover, should be celebrated on the Sunday following the
first full moon (14 Nisan) after the vernal equinox. Because of the
different methods of calculating the 14th day of the month of Nisan, the
date of Easter in the Western and Eastern churches is not always the same."

Now to confuse you.
*If ripe barley (Aviv) is found by the end of the Twelfth month then the
new moon expected to be visible on March 8 will be the beginning of
the new biblical year and about two weeks later will be the Feast of
Unleavened Bread.
If the Aviv is not found by the end of the Twelfth month then March 8
will begin the Thirteenth biblical
month and the Feast of Unleavened Bread will be one and half months later.
http://groups.yahoo.com/group/karaite_korner_news/message/342

This of course is the Biblical method of determining the first month of
the year.  This was not possible for many years,  so the convenient
method we use today came into use.
YBIC,
Glenn