Good and bad days -memory wise- why do I have them?

Depends . . . Depends on how old you are, your physical & mental health, stress levels, how busy you are, what medications you take, whether you drink, and what you have difficulty remembering. Poor health can compromise memory. Some medications can interfere with memory. Aging takes a toll. Do you forget important information that is well learned or more insignificant details. If worried, see a doc.