Therefore do not let anyone condemn you in matters of food and drink or of observing festivals, new moons, or sabbaths.These are only a shadow of what is to come, but the substance belongs to Christ.
And of course Gal. 4:8-11:
Formerly, when you did not know God, you were enslaved to those that by nature are not gods.But now that you have come to know God, or rather to be known by God, how can you turn back again to the weak and worthless elementary principles of the world, whose slaves you want to be once more?You observe days and months and seasons and years!I am afraid I may have labored over you in vain.
With the greatest of respect to Eric Lidell, whose story was told in the film Chariots of Fire, I believe he was mistaken about 'Sunday observance'. He refused to compete on Sunday because it was the Lord's Day, the Christian day of rest.
His interpretation might have been correct under the old economy, but even then the episode recorded in Mk. 2:23-28 casts doubt on that too. The point is that Sunday observance is not a legalistic issue or requirement under the New Covenant. We generally observe it because we want to, for our own sake, not because we are required to do so from a legalistic standpoint.
If you shop on a Sunday instead of going to Church, then arguably your priorities need to be reassessed. But if you shop after going to Church because you are expecting guests, for example, then you are not in breach of the spirit of Sunday 'observance' as set out in the NT, because as the Saviour reminds us, it was created for our benefit. It is there to serve us, not the other way round. We have not only shopped after a service, but seen other members of our congregation do so too, and neither side were embarrassed to be 'caught in the act'.
The same principles apply to Christmas and Easter. Perhaps even more so, coz pagan holidays adopted by the early Church as celebrations of Jesus' birthday or resurrection can have no real spiritual significance or NT mandates. They are just holidays, and if others choose to ignore them, they should not be criticised for it.