Set aside a certain number of days during which you shall be content with the scantiest and cheapest fare, with coarse and rough dress, saying to yourself the while: "Is this the condition that I feared?" It is precisely in times of immunity from care that the soul should toughen itself beforehand for occasions of greater stress, and it is while Fortune is kind that it should fortify itself against her violence. In days of peace the soldier performs manoeuvres, throws up earthworks with no enemy in sight, and wearies himself by gratuitous toil, in order that he may be equal to unavoidable toil. If you would not have a man flinch when the crisis comes, train him before it comes. […]
Modern phrasing: a deliberate inconvenience week. Plain food. No coffee. No music on the walk. Cold showers. Sleep on the floor. The point is not asceticism, and it is not virtue signalling. The point is askēsis — the rehearsal, the training, the inoculation. The fear of going without is much worse than the going without, and the rehearsal proves it.
Such is the course which those men have followed who, in their imitation of poverty, have every month come almost to want, that they might never recoil from what they had so often rehearsed. […]
Seneca, Moral Epistles to Lucilius Letter 18 · trans. Richard M. Gummere (1917)
Context
Written by Seneca during the Saturnalia, when Rome was given over to feasting and license. Seneca uses the festival as the foil for his counterpoint — practice the opposite, deliberately, so that when the lean times come involuntarily, you are not undone.