When things are calm. When you are surrounded by people you love and that love you. When your job is solid, and you have the respect of your colleagues and of your managers. When waking up in the morning is a pleasure, and you find out you enjoy things that you generally do not notice. When you have everything you could ask for, you are peaceful and willing to extend a hand to others in need. When the greatest worry is something you used to not even consider a while back.
That is the moment to prepare for harsh times.
It does not mean you should not cherish what you have, just that you should keep training for what you fear the most, so that when it will come it will not be as bad as it initially seemed. Eventually, you will find that the human condition is quite bearable, even in its darkest times.
[…] 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.
Seneca, letter 18