If a commandment is no longer observed as it was in the OT, then the onus is on you to show through Jesus' and/or the apostles' words how it was changed.
As an example, Paul calls Jesus our Passover Lamb that has been sacrificed. And he refers to the casting out of the unholy fellow from the midst of the Corinthian church as a 'keeping' of the Feast of Unleavened Bread (which comes immediately after Passover).
7Clean out the old leaven so that you may be a new lump, just as you are in fact unleavened. For Christ our Passover also has been sacrificed. 8Therefore let us celebrate the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth."-1 Corinthians 5:7-8
Law of Passover--fulfilled when you believe in the sacrifice of Christ.
Law of the Feast of Unleavened Bread--fulfilled following the Passover in the removal of sin from your 'house' (both, corporately and individually).
See how that works? Now that the Passover Lamb (Christ) has been sacrificed, clean the leaven (the sin) out your house.
You offer up the required Passover Lamb when you believe in Christ. And the cleansing from sin the household of believers receives when they believe in Christ is the fulfillment of the required festival to cleanse your 'house' of leaven that immediately follows Passover.
I can't think of any better way to celebrate and commemorate what God has done for us in Christ the Passover Lamb and the deliverance from sin in the Feast of Unleavened Bread
than to use the actual celebrations that God himself gave his people that illustrate those. We don't do that to be justified, we do it in honor and commemoration and thankfulness of what God has done for us. It's not for justification, it's for the honor of God in what he has done. No legalism, no attempt at justification. Just rejoicing and thankfulness before our God in a way that is approved by him and does not have forbidden pagan practices involved (sun worship, etc.).