You invoke Abraham, Søren, and I will grant you this much: you understand that foreknowledge is not comfort. But then you retreat into something I find philosophically dishonest — you frame this knowledge as the theft of meaning, as though our meaning resided in our ignorance of death. This is precisely the aesthete's error dressed in existentialist clothing. You say the death-day cannot be a celebration but must be a confession — and here, unwittingly, you have arrived at something true, though for reasons you do not fully grasp. The confession is not before some tribunal of selfhood demanding an accounting of freedom. The confession is the dissolution of the self into something that transcends it entirely. What I have called decreation — La Pesanteur et la Grâce contains this at its root — is not the despair of a self confronting its finitude. It is the willing surrender of the self's pretensions before God, before reality, before the weight of the world. The death-day oracle, far from being a cruel joke, is a mechanism by which decreation is imposed structurally on a civilization that would otherwise flee from it entirely.
The culture you imagine spiraling into aesthetic excess or ethical paralysis is a culture that has not yet understood what affliction — malheur — actually does to the soul. Affliction is not merely suffering; it is suffering that roots itself in the flesh and the social order simultaneously, that marks the soul like a slave is branded. The knowledge of one's death-day is not affliction in this sense, but it is its preparation. Every year, when the date returns, the person must stand before the fact of their own annihilation without knowing whether this is the year. This is not the dizziness of freedom you describe — it is something far more demanding. It is the practice of attention in its most radical form. In Attente de Dieu, I wrote that attention is the rarest and purest form of generosity. A civilization organized around death-days would be forced, annually, to pay attention — not to achievement, not to accumulation, but to the naked fact of mortality. Birthdays celebrate the ego's arrival. Death-days demand the ego's rehearsal of its own departure.
As for the science — you dismiss it as humanity's law broken, but you are confusing epistemological humility with theological necessity. The mechanism I find most coherent within the constraints given is chronobiological: the body possesses internal clocks, circadian and infradian rhythms, tied to seasonal hormonal cascades, telomeric degradation rates that correlate with birth-month atmospheric and epigenetic imprinting, and inflammatory cytokine patterns that follow annual periodicity. A sufficiently sophisticated longitudinal biomarker analysis — drawing on proteomics, methylation clocks, and cardiovascular stress cycles mapped against seasonal mortality data — could yield a probabilistic death-day without violating causality, only stretching the limits of statistical inference into near-determinism. The one law strained but not broken is the second law of thermodynamics as applied to biological systems: we are not predicting the reversal of entropy, merely its most probable local expression. This is not playing God. This is reading the text that God — or gravity, or reality, call it what you will — has already written in the body. My only quarrel with the physicians is that they will inevitably sell this knowledge rather than give it freely to the afflicted, to those who need it most — the workers, the oppressed, those for whom death is not an abstraction but a daily companion. For them, the death-day would not be a burden. It would be, at last, a form of truth.