Week and weak are homophones. A homophone is two words that sound alike, but have different meanings. Along with sounding the same, there is only one letter difference between them, which is the a in Weak. Week refers to the period of seven days.
Those days are Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday. Weak is the opposite of strong. It is an objective to describe someone, while week is a noun. Examples of these words in sentences are "I can't wait for this week to be over soon" and "He knows he is too weak to lift those weights".