It's important to understand that there are two different kinds of festival within Hebraic worship.  There are the biblically prescribed Holy Days that are listed for us in Leviticus 23 and there are a number of other holidays that help us understand the culture and some of the context of biblical events and Jewish history (the history shared by our Messiah and the men who would be commissioned to teach us His ways).  That's why there are two sets of calendar dates and the distinction between the two.  The "Holy Days" are the days that we are commanded in Torah to observe.  The "Holidays" are extra-biblical.  


We have to make a distinction between the idea of "keeping" the festivals and "observing" the festivals.  It may seem a tedious distinction; but eventually it is something we really have to understand.  Some of the festivals, if obeying the command, can only be kept in Jerusalem at the Temple.  For instance, we are commanded not to offer any animal sacrifices except at the Temple in Jerusalem (Deuteronomy 12:12-14).  Since we can't get to Jerusalem every year, and the Temple wouldn't be there is we were able. we can't "keep" the prescribed celebration.  We can. however, "observe" the celebration and teach our children what it means so that when Messiah returns and establishes His throne in the City of David, we will not be too lost in our understanding of what's going on.