Actually, the feasts were given to Israel to celebrate different aspects of their history, and relationship with God, who gave them deliverance and providence. We see then that when Israel celebrated these feasts it was not with the Messiah in mind or in focus, but upon what God had done for them. So, while they indeed are a shadow of what would be fulfilled in Christ, they were not celebrated for that function (that they served as; as a shadow).
Its funny how Christmas and Easter are considered pagan holidays in origin and how we are not to celebrate them based upon origin, but hypocritically those in the HRM want to celebrate God's feasts contrary to their origin. Israel celebrated what God did for them previously, not what God was going to do through Christ (at least intellectually). So for us to super-impose our understanding of these feasts as shadows (which they were) and celebrate them with this understanding, we would be celebrating them contrary to the reason they were celebrated in the first place (as a remembrance of what God did for them previously, such as Passover).
How do we celebrate a feast that looks to the past (a specific event), and reconstruct it to celebrate a future event (that is now past for us), because it was a shadow of what was to come? In essence, we are rewriting the feasts in a way, though we understand it as a shadow, we also have to understand how and why Israel celebrated a particular feast (and in remembrance of what). How do we change their reasoning for a feast, and Christianize it (just like the pagan holidays, according to Hebrew Roots Movement), and then act as if Israel didn't celebrate, for example, Passover in remembrance of what happened in Egypt and covering the doorposts with blood, but instead celebrate the feast in knowing Christ to be the realization, the Passover Lamb?
Is this not ignoring origin and attempting to Christianize a feast, though it be a shadow? I don't dismiss the importance that such feasts foretold of the work of the Messiah, but they were celebrating and taking part of these feasts for very specific reasons. How do we remove these reasons, and replace them, without the hypocrisy of not considering origin?
While I am not equating God's feasts with pagan celebrations, I am trying to make a point that to consider origin means that we must understand why such a celebration took place. To not "Christianize" it, as is often the accusation. Does this not occur with the feasts? Celebrated originally for a specific reason but then changed, celebrated in remembrance of what Christ has done, thereby taking a shadow and still celebrating it, even though the fulfillment has come to pass? Doesn't Communion then take precedence, and instead of changing the original reason for the celebration of a feast, shouldn't we rather partake in Communion?
We acknowledge the shadow, and it pointing to Christ, but do we reinterpret the shadow so as to be able to observe it today, when that which it foretold of has "came in the flesh"? Why do we put ourselves back under the Law? To me this doesn't make sense.