As I sort of semi-observe Lent, I've been holding in my mind two themes from the Apostle Paul. On the one hand, in Galatians, Paul frets over his converts observing "days and months and seasons and years"; he sees it as evidence that they are turning back from their profession of faith in Christ and returning to old pagan ways. I don't imagine that the Galatians are actually being tempted back into paganism. Common consensus is that they were just being encouraged to add some Jewish distinctives to their Christian faith. But for Paul it is all the same. They are turning back to slavery under the weak and beggarly elements of the world.
On the other hand, in Romans, Paul sees the observance of particular days as a non-issue. It is indifferent, in so far as it does not become a badge of some superior spirituality. If seasons are observed in honour of the Lord, fine. If they are not observed, because of the Lord, great.
Of course, in neither of these cases is Paul thinking of the seasons of the Christian year, which were centuries away from being thought of. His target is primarily Jewish observance, and some of his anti-observance rhetoric comes from his clear desire to maintain the truth that there is no need for Gentile Christians to become Jews. But the flexibility in his approach does, I think, point to something deeper.
For Paul, the important change in time and season is not in any annual round of fasts and feasts. For him there are only two times: this age, and the age to come. In Christ, the age to come has already invaded this age, and by the Spirit more and more people (even as they live out their lives in this age) are participating in the age to come. The decisive change in time has already occurred, and is now being applied through Spirit-empowered gospel proclamation.
So long as that central truth about time is not obscured, Paul does not care whether his converts observe yearly festivals. Perhaps that is a helpful way for us to think. As human beings, we naturally mark the passage of time. In some way, we are always going to structure the day, the week, the year. This is a natural phenomenon. But it can be pressed into gospel use, in so far as we relate our time - the thoroughly relative and relatively unimportant changes in the passage of time which we are compelled to mark - to the real time, the fulfilled time, the arrival of the age to come in Christ.
If I observe Lent to the Lord, as a way of remembering him, then I am blessed. If I turn it into a way of acting as if the new day had not dawned, then I am heading back into slavery.