When should you use the passé simple instead of the imparfait?

The "passé simple" is most often used in literary and formal writing express a completed past action. E.g. Un jour, elle tomba malade. (one day, she fell ill)

The "imparfait" on the other hand is used to describe continued or repeated actions in the past. It expresses what is going on at an indefinite time in the past. E.g. Chaque jour, Manon mangeait son déjeuner chez elle. (Every day, Manon would eat her lunch at home)