We often say, in our native language, that
Horses cannot fatten while grazing during the battle hours.
which means that one cannot prepare himself at a time when something is impending upon him. I will explain my proverb with another example; If someone has an exam and he starts studying one day or a few hours before it then we say that horses cannot be made full (grazed to full) at this time, meaning that you cannot prepare yourself when something is very impending on you. Is there any such proverb in English for this situation?